Yes, I think it is acceptable, with a sense of reasonable common sense, to "go after a guy." By reasonble common sense I mean to not make it an obsessive or over-the-top approach. If you call and he doesn't return your calls after maybe a couple of tries, he's not interested.
If you're standoffish and shy, you might appear stuck up. I watched this TV show a couple of months ago about "how to get the guy" and there are several things you can do with your body language to show that you're interested in someone. First thing is eye contact. Make eye contact and hold it for like 5 seconds. And smile. Smiling is very important if you don't want to be "standoffish." Then asking questions, spark up a conversation. Add in a little bit of flirting. Lean in a bit when you talk, casually touch his arm. If you don't get a response, the guy has a problem or he's not interested. I always find asking the question "so, are you single?" basically means I'm interested in someone. Why else would I ask the question if I wasn't interested? Many people will say that they aren't single (even if they are) just to show they aren't interested. Of course someone might say they are single and still not be interested, but you can usually gauge by their body language and such.
But, guys are just as afraid of rejection as girls (maybe even more so). So quite often we ladies must take the lead and approach the guy. I think in general it is often easier for women to pick up a guy than vice versa, at least that's what I hear.
Plus, many guys are attracted to assertive, confident women. However I'll say some prefer the shy type as well. So, just be yourself, make eye contact, try to flirt a little and see what happens from there! Good luck!