Most engines will keep a slight pinch in cold conditions, it has to do with expansion of the piston and sleeve on worktemperature and helps to have a cold start. When the pinch is gone and the engine runs fine then there is nothing to worry about.
When the pinch is gone and you have running problems then yes, the set must be gone....
A nice test is to grab the engine at its flywheel and let it hang on its compression:
If it stays hanging the engine is very good
if it goes through its compression within a view seconds it must be fine
if is falls directly through its compression and the engine runs bad then you need a new p/s set.