That is sort of correct. Garage queen Boxster's are more likely to have problems because they are not driven enough, and seals and things dry out and crystalize, etc. Cars that have been driven regularly (hence more miles) tend not to have these types of problems because they have not been sitting around. Just another testament to the fact that you should drive your Boxster, and not keep in the garage.
On older cars (pre-2001) IMS is very, very rare and you really only have to worry about RMS which can be detected ahead of time by watching for oil leaks. In '01 Porsche redesigned the IMS (to eliminate vibration/noise) which made it prone to failure, the older cars didn't have this problem. So with a low mile pre-'01 car you would probably not have to worry about IMS. After that year, you can be concerned about both RMS and IMS.
Generally, after 35-40k miles the chances of either IMS or RMS decreases, but they can still happen. So buying a car with 68k miles vs. a car with 40k miles is not going to make much of difference in safety against IMS or RMS.
Good Luck with the Decision!
