I have done it. The ones I've re-done (because they were not good or because they were prelims) came back tighter when taken at another time. Changes in the bone will look exactly the same. I also did one 3 weeks after weaning the puppies and they were way out. I re-did them and they came back good.
If you decide to do them and they don't look good, re-do them; do not take that as the final say on the hips of your girl.
The hips are NOT more lax when a bitch is in heat! There's a reason that the canine pregnancy test measures levels of RELAXIN in the blood, which is only present when the bitch is pregnant! This is the hormone that causes laxity in joints. My repro vet has no problems xraying a bitch in heat - I've done it several times - Excellent hips each time!
I have to agree with most posters here. My girl came in season a week after the x-rays were taken and she came back Excellent Hips and Normal Elbows. Of course, I had an x-ray tech who knows what she is doing :)
