Saving money - to book rooms online or not?

If your looking for bargain cheap as chips, then you should wait til you get to the bus station at a city and negotiate with all the touts for the best bargain room.

In Lima, you won't get that so book online for that.

20-30USD per day is tight. And not a lot. Most hostels were about 10USD for a dorm bed.

Uber is ok in Lima. Never used it elsewhere. Taxi fares should always be negotiated and agreed on before getting into the taxi.

Does anyone know if its better to book online or to negotiate in person when traveling peru?
is there ever a situation where the online price is a better deal than just going in person and haggling?

For most places in Peru, you'll get the better deal if you wait until you get there and walk around to find cheap lodging. The cheapest places aren't online via booking websites. And the private rooms at tourist hostels always cost more than local budget hotels. If you negotiate, you might get a few dollars cheaper, but it's not a bargaining country like India.

Still, I would book a room in advance for places like Lima and Aguascalientes.

Definitely book ahead for Lima, it's a dangerous city and you need to know where you'll be going when you arrive. If you want to keep to that budget you'll have to get out of Lima too, in smaller places like Iquitos it's quite possible to get by from 20-30 but if you want a private room 30 is more realistic, 20 in the dorms.
You might have to do a bit of searching to find good value rooms, I didn't find the best rooms until my 4th or 5th day in Iquitos.

There are many local hotels that will be better value than places on Internet that cater to foreigners.

Another tip (although maybe more applicable to mid-range hotels and up than hostels): contact the hotel directly rather than book through booking websites. You will almost always get a cheaper price for the room and you usually won't be required to pay anything ahead of time to hold the room. This makes it easier to change your plans on the fly. Look at hotel website and then send an email directly to hotel to book accommodation. We didn't have to pay for a single room - not even a deposit - ahead of arrival and every single reservation was honoured. Booking websites take a fee for their service and so rooms are generally a bit more expensive. And cancellations are often difficult or impossible unless you do so quite a few days ahead of your arrival.

From my experience you don't need to make bookings online. I've never booked any accommodation in advance anywhere in Peru, or in South America for that matter. The only exception was when I arrived in Lima late at night and jet-lagged. I booked either at Hostal Barranco or Alison, both in Barranco. Also Casa Ana in Breña, although that area is a bit boring. The guy running Hostal Barranco offered pick-up form the airport at a decent price.
Normally, you can check a couple of hostels in the same and neighbouring streets, try to bargain and choose whichever suits your budget.
I wouldn't say Lima is a dangerous city, as one of the posters claims. Not more than any other big city in Peru or South Am. Like everywhere, certain areas (Rimac in Lima) should be avoided, at least at night.

Not true, so stop spreading disinformation. Lima is known to be a very dangerous city where tourist are limited to a few neighborhoods. Granted, if you stick to those neighborhoods during the day, you're pretty safe but Lima is not safe.
Quoted from US government info for 2017:
https://www.osac.gov/Pages/ContentReportDetails.aspx?cid=21169
THE U.S. DEPARTMENT OF STATE HAS ASSESSED LIMA AS BEING A CRITICAL-THREAT LOCATION FOR CRIME DIRECTED AT OR AFFECTING OFFICIAL U.S. GOVERNMENT INTERESTS.

Armed robberies, assaults, express kidnappings, carjackings, burglaries, and petty theft are common in Lima. While gratuitous violence committed against foreigners is infrequent, assaults and robberies involving violence have been increasing over the last several years.

I guess all regulars on this branch have visited Lima at least once. You're the only one that all of a sudden comes up with such a warning.
If I'd have to follow the US government advice my choice of destinations would be very limited, and I'd better stay at home.
Relax and happy travels.

I visited Lima too, and I remember all the warnings I got from other people, telling me to strictly limit myself to the few tourist districts. When I tried taking the public bus from the airport I remember the shocked look of a kind old lady the way she tried to warn me that I should not do that. I remember the cab driver, who drove though Callao, asked to lock all his doors and pull up the windows when driving through. I also wanted to walk around Callao a bit, but after looking out the window it was pretty obvious I would be mugged in less than ten minutes. The couple days I was there four masked gunmen attacked some big shot in a yellow lamborghini and shot him and the lamborghini to bits with machine guns. This was in downtown Lima in broad daylight with all the people going about their business. Apparently, just another day in Lima. So no, Lima is not safe. I'm not a beginner, I've traveled and lived extensively in Asia. As the advisory says, violent crime has increased in the past few years. They don't just make this stuff up.