how crucial to plan ahead?

You can do it on the fly down there,but I doubt you can do it low budget ,Cuba is not inexpensive and it's one of those places that unexpected expenses can easily happen,make sure you keep a back up stash of cash.

You can always ask where you are staying, where they would recommend to go next.
We have been to Cuba many times and never had an itinerary.

Hi: I have been to Cuba many times. We find the Viazul to be the most comfortable transportation next to hiring a driver. There are cheaper ways to get around. You could pay 2 cucs each to ride in a stand up open truck.... or there is also another bus service that is cheaper than the Viazul. Keep in mind that you have to book a day ahead with the Viazul bus. Apparently, you can now book on line which will save a bunch of time where you would otherwise have to stand in line. I have never booked on line however. Speak with your hosts they are a wealth of information and love to help. They could also call the bus station for you to book your tickets if your spanish is not up for such a conversation.

Our best adventures start out with no plans.... You can be as low budget as you wish. Beer is a buck....you can buy a 6 " pizza for .25 ..... breakfast at your casa will be anywhere from $3. to $5..... Dinner from $5. to $8. depending on if you have chicken or lobster.... most casas are $20- $30. most.... $25. depends on where you are.

Have fun.......What airport are you landing at?

Two CUC to ride in the back of a truck? Maybe for a trip from Vinales to Santiago de Cuba.

We were in Cuba the same period last year. Accommodation: not a problem at all. It sometimes took 2 or 3 phone calls to find a casa in the next place we were heading to - but generally not a problem. I think transportation is another matter though. We had a car (booked 3 months in advance) but we met (and gave lifts to) several tourists who were despairing because Viazul was fully booked - including a couple we met who were getting the Viazul in the middle of the night to somewhere they didn't really want to go as it was the only seats they could get to anywhere at all in the next 3 days. You might end up having to get taxis, which is not cheap.

If your time is limited to less than 2 weeks, planning and booking ahead is the way to go. Anything else and you risk blowing up your holiday in frustrating queues. Demand outstrips supply for both accommodation and transportation so do make a detailed plan for your journey and book in advance everything you can. This is particularly important for small places that score high on visitors wish list, typically Trinidad and Viñales.
Viazul: apart from limited availability, their booking system crashes on a regular basis. If this happens, they will refuse to sell you a ticket for the following day. You will be asked to show up an hour prior to scheduled departure and hope for a miracle. By booking your ticket online you beat the system. Your ticket is confirmed and reserved, whether their system is working or not. All you have to do is to hand over the printout of your confirmation and exchange it with the actual ticket. Even changing money can be a time-consuming operation. Calculate the total amount of money you will need for your trip and keep Cadeca (exchange office) visits to the absolute minimum.

Sorry for the confusion regarding 2 cucs for the truck ride. Some ladies I meet took the truck, it is not a cross island deal. They rode from Holguin to Herradura. I would liken it to a cattle truck. You don't want to do that unless of coarse you are looking for the cheapest mode of transportation.
Happy Trails

Yes even Cubans hate using the dreaded camiones to get around,more frequently used outside of Havana.

Hi, thanks! We fly into Habana on the 22nd, then rent bikes for 4 days. Most cities are booked on Viazul for the 27th, so it looks like it'll be taxi, stand-up truck, or ... hopefully train. But I imagine the train(s) will be booked as much if not more than Viazul, and they only run every other day or so. But a train ride would be awesome. Too bad we're going during the busy holiday season.
Would love to hitchhike, taxi, whatever, but others are saying it'll either be expensive or hard to find rides last minute. I don't know ... I'm not worried about spending money, but my partner is.