I agre with Rob and Anjay, but I think 5 months in advance would be far too much.
As he is looking for a cheap server, I doubt he wants to have a live server for 5 months while he is paying for it, but not actually using it.

Buying one month in advance might help you to ensure it is setup and secured properly, but if you are trying to save as much money as possible, try wait for a good deal then go for it, when you need the server.