As far as I can tell, it doesn't stress the computer much at all.
Just out of curiosity, what's the longest amount of time you've left Fusion running? I can't seem to get it to continuously run for much more than 5 days before it loses "exclusivitiy" to the Bootcamp partition and quits.
I'd have to question the idea of limiting how you use your MBP to control the temps. If your MPB gets too hot from any kind of normal use it should be repaired. The question I would have is whether keeping Fusion running all the time slows down what I'm doing in OS X.
Run the Activity Monitor to see how many resources Fusion is using.
I'm pretty sure that for anything other that working with photos/videos/animation, the MBP can handle having VMware and OS X running.
I haven't noticed a dip in my day to day activities.
Also, something I just noticed is that Quicksilver recognizes programs in your VMware (at least with Boot Camp partitioned). I can launch Office 2007 through Unity nearly instantly with Quicksilver. If only I could automatically open docs through Office 2007 that are on my OS X desktop...