I check for the changes implemented in any software and see if the new feature set and/or fixes provide benefit to our development team. In the case of FogBugz, I have not had a single problem caused by the upgrades. Fog Creek has done a remarkable job at testing their product before it hits our system.
I am not saying FogBugz is perfect, but it is close. I cannot say this about every software vendor. I really dislike when software installs screw up the workstations or the server. I use to be a jump on the new release as soon as it ships kinda guy. But too much blood loss being on the bleeding edge these days so I now wait a little bit and see who else has success or failure.
By the way, this is a business decision, not a technical decision. And I make it on a case-by-case basis depending on the reliability of the vendor and the level of importance the software makes to my company.
Rick Schummer