I never was the type to go out specifically hunting for female attention, but I know a lot of guys are like that. My wife, and every other cool girl I know, can't stand that kind of thing.. but then again, many girls like aggressive / "confident" guys, from what I've heard. I don't get it, but hey, different strokes or whatever. As long as you don't harass them, everything should be fine.. but as others have mentioned, a lot of guys do keep pestering their "prey" and won't take no for an answer, so it could just be easier to give you the number and blow you off later.
All that aside, calling them up immediately after you get their number comes off as really rude. In a purely logical world, it wouldn't be.. after all, why should you have to trust someone you just met? Unfortunately, people have feelings, and want to believe that you're attracted to some kind of "goodness" that you sense in them, and not just their physical assets. Or something. I really shouldn't even have to explain it.. you know what I mean, I'm sure.
Whoa. Every sentence in my first paragraph follows the formula of "X.. but Y". I am seriously slippin' in quality control. Also note: "excessive" use.. of ellipses and quotation marks.