Commercial cheddar producers will put up to 8 psi on it. To be fair (as I'm always ranting about :-) ), pressing pressures don't belong in general recipes (again IMHO). There is enough variation in makes that the amount of pressure *you* will need is almost unrelated to what the author of the recipe needs. Having said that, you can't over press a cheddar. If you press too hard, too early, you may end up with some mechanical holes in the middle as the air wasn't allowed to escape, but it's not a really serious flaw. If it's not closing, you just need to keep upping the pressure until it does. If you run out of things to pile on top, then you can try to heat up the cheese (to 30 C, for instance) and press it warm, but that's a last resort.
I usually make a Caerphilly instead of cheddar because you cheddar for less time, half salt the curds, and let the cheese slowly acidify in the press. The higher pH allows the curds to knit more easily and so you need a *lot* less weight. Then you finish salting it after the cheese has acidified to the level that you want.