Most Helpful Guy
If the guy has any class whatsoever, you dressing conservatively is not something he should view negatively. He either likes you or he doesn't, and how you dress should not affect that.
Keep in mind, that you can still dress nicely even if you're dressing conservatively.
The only thing that would annoy me a little bit is if a woman dresses in such a way to hide her figure and natural beauty. Either that or if she dresses really sloppy or only wears really drab stuff. But these things are minor fashion annoyances, and if I like a girl, I'm going to like being with her regardless of how she dresses.
In some cases, dressing conservatively is less frustrating than someone who shows a lot of skin, because it teases the eyes less.