Well... I live in Germany, and I always have a dynamic IP, but like AceJay said, it's not COMPLETELY different.
From Monday to Friday, I'm usually in another town, connected to another network/internet, so my IP changes. Voting is no problem, since the timer sticks to your forum account, no matter which IP you are using.
I voted from both IPs, the one I have now, and the one when I'm at the university. Never got problems, and the timer always worked.

As long as you don't use two different forum accounts (which is forbidden anyway), you're fine.