Videos of people on the ISS make it clear how little they need to bend or stretch, and how their joints stay in their neutral position most of the time as they float around - elbows, knees and hips slightly flexed, shoulders lifted towards their heads. It seems to me that if they didn't specifically stretch to ensure that their joints don't tighten up, by the time they got back to Earth they would have lost a lot of flexibility.
A quick search doesn't show anything in particular in the ISS exercise routines regarding stretching. Is it something the crew just does on their own now and then, as the urge arises? Has loss of flexibility ever been noted?