For me it usually comes down to how hard the part is to get to. Plugs and wires I usually just get the generic cheap ones, but then again I replace mine often (every other year about) just for the piece of mind. Usually my guideline is: if I don't want to have to do it again, I get a toyota part, if its something I do a lot, I go generic. Things like hoses (pcv, idle air, etc) that are on the top of the engine I feel should definitely be toyota even though they are easy to get to for several reasons:

1) The ones on my truck were 22 years old, and still in good shape so I figure these should last just as long.
2) Perfect fit means less skinned knuckles installing things.
3) Only cost a few bucks more, granted if you do a lot of parts it adds up.

One thing you can be sure of though, if you go with toyota parts you won't regret it, with generic parts it could go either way.