My script automatically detects proxy IPs and flags them, but I don't worry about non-proxy IPs changing. Many people move around or use an ISP that gives them various different IPs every time they log on. I know that my IP address will float since I'm on DSL, and I get a different IP every time I turn my computer on (they're all in the same range). If I see a proxy IP that's in the same range as the actual IP (I see both for every submit) I figure it's someone on DSL, AOL or something like that. When I see a proxy that's a completely different range it tells me that the submitter is trying to hide their location.
In the end I'd say don't worry about it
