I suppose I could just say "here's $10,000, do what you want" and that would be a gift. No further say in the matter. But if DW and I, and maybe the groom's parents, are "hosts" of the reception, I'd consider it an expense of throwing a party, not a gift. I'd also give a separate "gift," such as paying for the honeymoon, or covering some other cost of the wedding: flowers, photographer, DJ, etc. Or maybe buy them some furniture for their house, something along those lines.
By hosting the reception and paying for it, I'd want some "control" by saying I get to invite these 6 or 8 friends of mine. If they wanted an entirely vegan meal, nope, no dice, you need at least one meat dish or at least something with cheese, like veggie lasagna. No three hours of Death Metal. You can have some, but there has to be some softer music to suit more sensitive guests. It's not ALL about you, bride and groom, if someone else is paying. You have to take other people's tastes into consideration.
Black roses and White Castle are fine.
(If I had total control of the music, it would be mostly three hours of late 70s/early 80s New Wave, with some punk and Goth thrown in.)
OK, I confess I have no idea what a rehearsal dinner really is. But I did think it was only for the bridal party and the bride & groom's parents. Who gets invited to account for 40-45 people?
I was a best man once, and a groomsman twice, but I don't recall any rehearsal dinners. Maybe I wasn't invited.