I won't comment on if it's proper or not, mainly because I just don't care!
...and I truly don't know. I do think it can be different based on locale, nature of relationships w/those invited, and culture.
You want what you want! Right?
Okay, so if I were doing this I might go w/setting up an account at a hometown local bank to be available to those who'd like to give you a small amount and yet it remain unknown to you WHO deposited what amounts. I wouldn't want someone to be embarassed if they could only give a little, KWIM?
I'd also have a money tree available at the shower and any reception or party w/lots of little ribbon-ties to be used available for people.
I'd also let people know that this option was going to be available. But, how to??
Hmmm...I'd have whomever was hosting any shower to include a signed note (okay print it up, but do have them hand-sign each one) that read something like this...
and on the wedding invites too, I'd have my maid of honor and best man sign these inserts...
"Ms. Cooliemae & Mr. Cooliemae-To-Be plan to buy a home soon to get them started in life. Although registry gifts are lovely they already possess many household items, so, their family and beloved guests are encouraged to gift them w/cash or equivalent in lieu of a wrapped gift to help them with their future down-payment. A money tree will be available at the shower/reception or you can make a deposit at $$Bank in downtown Cooliemaeville to the "Cooliemae House Fund". Every time they enter their Home-Sweet-Home they will be reminded of the great send-off you gave them!"
...or something to that effect!