It depends on the current draw of your tomtom device and whether your USB port is for charging and reading music files. If it is only for reading music files, then it may not have the capacity for charging/powering an external device - ending up with a blown fuse. But I suspect the tomtom should be fine.
Why don't you use the 12V cigarette lighter instead? Most satnavs are powered from the cig lighter.
If you use the tomtom a lot, it may be worth considering having this hard-wired into the car to free up your 12v cig lighter and have a neater install - rather than have wires dangling everywhere.