Buy in advance if you can, but don't sweat it if you can't.
Here are the reasons:
1. Certain bus times are more popular and can get sold out.
2. If you buy ahead of time, it can be cheaper. Some of the bus lines advertise a $1 ride which you might be able to get if you buy a month in advance.
3. Buying online (for Bolt Bus) can get you frequent traveler rewards. Even if you don't ride it often, buying your ticket through an account on Bolt Bus will get you into boarding group A, which means you are among the first to board.
4. If you buy it online and miss your bus or decide to take an earlier one, you can probably still get on the bus provided it isn't completely full.
1. Arrive about 15-20 ahead of schedule, but that's definitely playing it safe. Many times, the bus isn't even there until 5 min before departure.
2. All the buses have terminals at South Station, but it's a few minutes walk from where the subway lets you off. Plan for enough time to find the bus terminal.
3. Bolt Bus and Megabus let you off in central Manhattan, whereas Fung Wah drops you off in Chinatown. Both stops are near subway stations, but depending on where you are going, it might be easier/faster from a specific drop off.