Go for it. If they were going to leaf out, the stems would be green inside. "Late" stems will leaf out as late as sometime in May; end of May if you are in the northern states. This year, I had some stems that leafed out and others that did not but right after the end of May, the pruners called me and the rest is history.
Even when they leaf out, you may consider pruning them (enjoy the blooms for a few weeks and then cut) so you do not end with a plant where a few looong stems are sticking out with leaves and the rest of the stems are smaller. It will look strange. The best time to prune hydrangeas that bloom from old wood is the end of June (you probably can extend that into July if you are up north but be wary).