This is happening quite a bit. Good cows are hard to find, so people buy exposed cattle that aren't pregnancy checked because they know the next looker will buy the cattle, without being checked. So, you take a chance and buy them before the next guy and see what happens. The information you do not provide is if you sell the open heifers, how much will you lose? If they weigh around 1,000 lbs and open, they will bring around 1.30-1.40 per pound, here. So, it depends on how much you will lose. If you will lose too much to stomach, then keep them with your bulls and sell them as springers or keep them, when the time comes. If you won't lose much per head, then sell them now and put the money into something else.
One thing to remember is that if the bull was put in close to Aug. 31, then the first breeders are only 3 months bred and most of the time they have to be 6 weeks to 2 months bred before they will preg check bred. So, if you sell now, you may be selling several short bred heifers, even though you are sure that 10 have bred since you got them. Good luck in your decision