"Wasting a girl's time" ... I think this comes down to two issues: your approach (direct vs indirect) and knowing what you want from a girl.
Your approach (direct vs indirect)
It's hard to discuss "wasting a girl's time" without getting into a direct vs indirect game debate. If you're looking for a fuck buddy then by using an indirect approach, you are more likely to mislead the girl. If you're lucky, her intentions will line up with yours and everything will work out, but if she sees you as a potential boyfriend and feels that you are "courting her", then they won't. If you're direct about your intentions from the get go, then you won't have this problem, although you might end up potentially "leaving some pussy on the table".
What it comes down to is personal ethics. If you feel the only way you can get in a particular girl's pants on a regular basis is by treating her like she's your exclusive girlfriend and you can sleep well at night and handle all the drama that might come with it, then go ahead and be an asshole. If misleading a girl makes you feel guilty, if lying to her makes you uncomfortable and you'd rather be authentic about your intentions, then you might be better off being straight with her from the start and accepting that you're just going to have to leave alone some women.
Knowing what you want
The other issue is knowing what you want. I might see a girl as a fuck buddy, only later to see her as a potential girlfriend and start treating her that way as a result, only to later decide that I just wanted her as a fuck buddy after all. This would cause me to end up "wasting her time", even if I was trying to be honest with her from the start. Unfortunately, this is something I'd have less control over.