Just leave it alone if you like the look of them - the flowers will do no harm. When they're over you could prune out the flowered shoots if you don't like the look of the dead heads and use the cuttings in the kitchen.

Sit and admire them if your indoor potted rosemary is happy enough to flower! It's tough enough to keep alive overwinter and in pots.

In general, the best time for herb flavor is just before flowering. But that applies much more to faster growers like basil, thyme, or oregano than woody rosemary, sage, or lavender. I don't think you'll notice any difference in flavor--and the flowers are edible as well should you choose.