Having several servers with 256 Gb memory . Played a lot with the memory limits parameters (70/90/10), noticed nothing.
Assuming that your QVW files are correct (no synonims, working formulas in charts) and contain no useless columns and rows. If you expect 100 users, there is no point waiting until somebody complains, than restart the service manually.
My solution is to automatically kill and restart QVS when using too much memory. You can measure statistically above what memory usage the QVS does not work anymore / does strange things as in your case.
The issue has been solved removing the parameter ClearCacheTimesPerDay on registry.
I tried the parameter to limit the memory growth
the parameter isn't officially documented but I found it in this post (http://community.qlik.com/forums/t/30238.aspx)