As 889 says, it really depends on what tickets you are buying and how crucial to your plan there are.
You can usually take a slower/less ideal train or rough it on a bus or pay extra on the high speed rail if things sell out.
I seem to remember you being cost conscious from a previous post, but dont remember if you wanted the cheapest
Hard seat trains or to make use of hard sleepers overnight. Hard sleepers sell out and they tend to be few in number. The cost difference between the ideal sleeper berth from A to B and getting HSR or the bus from A to B during the day and a hotel room is huge, so booking in advance is worth $5-10 easily.
If you are planning on taking the HSR during the day, it is less likely to sell out and your options if it does sell out are to take another HSR at a different time or the bus which takes longer, maybe that isnt worth paying extra to book in advance.
Buying tickets in person in China is a pain, even if you speak Chinese, so it might be worth it anyway to book in advance through an agent/website.