Just make sure they have not been stretched in any way. Compare the lengths with those found in the Factory Service manual. If they are within specs, they can be reused, otherwise, dump em. BTW, your rotors should have come as a kit with new bolts, just as new pads, should come with new shims and grease, if purchased from a VCNA licensed dealership.

To be certain of proper bolt length, you can use the ones you ordered this time, to compare with the old ones. If the old ones are still good, use em, and keep the new ones for next time.

Be SURE to torque everything down to specifications. This is one place where, "by feel" won't cut it, especially on Volvos.

If it helps to reassure you, EVERY Volvo tech ALWAYS uses the old ones again, UNLESS, they find visual signs of the bolt being stretched, or damaged in any way.