I agree, I think it's fine and sensible for parents to prepare ahead of time, especially for the big things that they know they want and that tend to be a bit expensive for baby shower guests.
I do think you might consider waiting until after the shower to buy baby clothes, small toys, books (aside from those that are special to you), and other smaller baby items with a big "cuteness" factor. That way, you know that your baby got a *ton* of cute little two-piece sets and fuzzy sleepers but no onesies and you need to buy those, or tons of little toys and you don't really need any more of those but no books, so you'll want to get those. Etc. You might consider waiting and registering for a pack 'n' play, if you want one, as that's a "big ticket" item that is still cheap enough for people to sometimes want to go in together for it. Same with baby swing, bouncer, Exersaucer, things like that (in the $40-100 range). It would depend on your crowd, but sometimes family spends a bit more, so they like those mid-range items, or they might want to go in together, so having a few still available is nice. Just because you're setting aside the money doesn't mean you need to spend it all now, you can also set some of it aside to spend after your shower.
I do regret waiting too long to buy our carseat/stroller combo, though. We *loved* this one set, and it was discontinued. By the time we looked at it again, it was impossible to find, and we had to settle for one that I never liked quite as much. Still makes me a little sad, as I ended up looking at the one we got for years and years, given that we're on baby #4, and I would have preferred the cheerier look of our preferred set.