If the cut end is facing up- like the top of a post- you should protect it somehow. If it's a 4x4 post, the easiest non-chemical protection is a post cap.

Otherwise, if you're talking about raised beds set in soil, or something like that... why bother sealing? Placing pt lumber in direct contact with soil voids the warranty because you're encouraging rot conditions. I find it's easier just to accept that any wood making contact with the soil is going to have a limited lifespan, and will need replaced at some point. Just buy the best quality lumber you can find. I'm appalled at the garbage Lowe's sells- it's green, it's sopping wet, and most of it's already cracked and twisting. The #1 select sold at my local lumberyard is the same price, but it's actually nice quality and good looking stuff. Lighter, too, since it's had time to dry. Since I just built my fence by myself, lighter weight was a huge benefit.

The sawdust from cutting PT lumber is supposedly a risk to breathe in; and for later, 5-10 feet is nothing in terms of chemical travel; consider that roots cover some distance even if you don't. Besides, even if you don't grow vegetables at that spot, someday you or someone else might, or you might end up moving dirt around.

I have ended up with the same approach as Marcinde - if it's wood, it's only going to last a while, so I don't even try to make it permanent - for example, we don't put posts in concrete. But that 'while' can be 10-15 or more years even with non-treated wood if your drainage is good... so the answer to your last question is probably yes.

KarinL