With or without straps? I'd probably go for with, as long as they're not loose or falling off your shoulder, otherwise strapless. If it's a see-through top or otherwise very "airy/open" (exposing your bra), make sure it's a nice looking one, not browny/nude colored. Above all else, please don't wear the clear plastic straps, they look awful. Well, that's my opinion, anyway.
Depends on how badly you need the bra... if you're bustier then you should definitely wear the bra. Just try a bra with the same color straps as your dress. If you're not as well endowed as some of us, and your boobs don't look... awkward without a bra, then by all means... roam free! =P A strapless bra or bras with clear straps are also an option.
I would say buy a strapless, or if you don't want to spend the money go out and buy those "sticker bras" or whatever, those little things you can just stick on your boobs for one time use. If you can't do either of those things, wear a bra with straps, and maybe wear a cute little half-jacket over your shoulders.
If you have to wear a bra you can try the kind with clear straps. That way you have support and your bra doesn't show as much. ^__^
A long time ago I found one that has a clear back strap AND clear arm straps! I love that thing.
If you don't want to buy a bra you can just buy a pack of clear straps (I've seen them sometimes at the dollar stores and bought seven packs from there lol) and attach them to the bra you are wearing with safety pins or something.
With or without doesn't look "tacky", at least to me. I say go with a strapless though it just tends to look a bit better and you don't have to worry about the straps possibly falling off your shoulder or something. Or maybe just get a bra that compliments the color of your dress
If you're not very busty (a cup to small b cup) you can probably get away without a bra! If not, maybe wear a bra and also a shrug or light jacket to cover the straps. But a strapless bra is a great investment in the long run.