I'm rather careful about possible memory leaks when coding, and at least in my experience this does not happen with Mappero (at least, not in an easily measurable amount), even after 6-7 hours of GPS tracking.
But indeed I've also noticed some slugginess, and it seems to me it's happening more often when you have been using the N900 for some days (especially if browsing). It could be due to the kernel being busy swapping processes in and out; and since Mappero is a memory hungry application, it's easier to encounter this behaviour when using it.
I recommend rebooting the N900 once every 3-4 days -- but it depends on how and how much you use it.