I reuse all the old bolts and nuts on the rear suspension. The only time I
won't reuse them is if they are too rusty or I had to destroy them to get them
off. There are a few spots where you are advised to use new hardware. The
four little bolts that hold the ball joint plate to the strut is a place
where Volvo advises to use new hardware. I've been fine using the old bolts
there but it is a good place to use new hardware.

The only spot I've found where it's been absolutely a must to use new
hardware is when I have the turbo or turbo exhaust manifold apart.

The rear suspension bolts are coarse metric thread. I don't remember the
bolt grade. Probably grade 10. Just be sure to use lots of PB blaster and
give it plenty of time to soak. A few days before you do the work helps
a lot. Especially for removing the rear trailing arm bushing. Usually the
only bolt on the rear suspension that gives me any trouble is the lower
shock bolts. That's another one to soak for a good long time. A breaker bar
or impact driver is useful for that sucker. This is one of the best things
you can do. It will give you that nice new car feel to the suspension.
Wish you well.
Dave 82 242ti