I have a steel frame with 132.5 mm dropout spacing, the manufacturer claims it is compatible with 130mm or 135mm hubs...but I wonder if it will remain compatible with 130mm hubs AFTER it has been run with 135mm hubs? My concern is the 135mm hub will stretch the dropouts so it will no longer "work" with 130mm hubs. Should I be concerned?
Short answer: Don't worry about it!
Steel is inherently a springy material and you will find the dropouts spring back to their original location once the wheel is removed.
In order to permanently re-set the dropout width, you need to bend the steel past its elastic limit. You can look up "cold setting" by Sheldon Brown or whoever you like if you're interested.