i'd say it depends more on the shape of your boobs than the size. I'm a 34D with really round, perky boobs. They're the kind of boobs that are high but seperate if you get what i mean (so i don't have an arse-cleavage look) and because of that, boobtubes and strapless things look really good on me.
i have a friend who's a 34C but her breasts have the 'stuck together' look, so she has a massive cleavage and boob tubes look horrific on her because she just has this line of cleavage looming above the top. looks very weird.
so i'd say don't worry about the size of your boobs, assess the shape and how they actually look in it! Also, if your boobs are perky like mine, you probably don't need a bra because boob tubes are quite 'supportive' anyway - unless you have bullet nipples, in which case definitely wear one!