After getting the cassette reel serviced, I notice (weeks later) that the steering wheel is sightly off-center (i.e. when driving straight, the steering wheel is ever so slightly at a tilt).

This is hardly a problem as it doesn't affect anything about the drivability or safety of the car, but I was wondering if this is something I can adjust myself? Or is this something that's hard to do and should wait until it's in the shop next?

personally i"d have an alignment done
when reattaching steering wheel after the cancellation casette has been replaced if it wasnt put back on exactly where it was previously
then it would be 1 or more splines off
being off 1 spline you"d of noticed as well as the tech would have that the steering wheel was off center more then just a tad/slight bit
best bet get it aligned

Apparently, this isn't something a man with a torx screwdriver can do, eh? Oh well, it's not a big deal....