you should make reservations as soon as possible. Most restaurants won't fill up until a couple of weeks before the date (with a few expections) - but the earlier you make it more likely you can get the time you want. For example, I tried to make dinner PS at teh Plaza (MK) about 70 days out for 5:30 and had to settle for 4:50. On the other hand, a 6:00 PS at Whispering Canyon was made about 20 days before we left.
It used to be that only hotel guest could make PS, but now it is open to everyone, so you really need to get it early.
Remember, there is no cost (just a couple of exceptions) and you can always change/cancel them. YOU DON'T want to be walkng around EPCOT at 5:30 trying to find a nice cool place to eat. Go ahead and make a PS as a backup if nothing else.