Y'all, I am just so incredibly tired of the whole "but no one is required to participate!!!" argument.
There are lots and lots of things it is tacky to even ask people to do. You also should not ask people to let you keep the plate when they bring you a dish, and you should not ask people to buy you dinner after they help you move. Yes, they aren't required to accomodate your request, but the fact that you aren't forcing them to do it at gunpoint does not make it more polite!
Bottom line, if you wouldn't feel comfortable making the request face-to-face, then don't write it on your invitation. If you do feel comfortable asking everyone face-to-face, then you have my blessing, I guess. But be prepared for the look of uncomfortable shock when you ask me to take out my wallet at your baby shower. (And if a MTB does feel comfortable asking in person or on the phone, then the whole "I want books but I don't wanna look gift-grabby!!!" thing no longer flies with me...)
PS - OP, this is in general and not about you; I don't think you used this argument at all.