I rode my mare the day of her last breeding date in a Cowboy Mounted Challenge and she took and foaled out just fine. It wasn't light riding as well since it was hot and it is a timed event/all day long. I think the difference on whether you can ride her or not is whether you're riding her right now and she is in shape. Although, I would hate for you to ride her and have her come up open or something. It is completely your choice. A friend of mine took his mare to be AI'd at the vet's office, he was roping on her that afternoon. The vet said as long as they are in shape, it doesn't hurt them either way, but different vets might have different opinions.