Why does the steering wheel wobble slightly at any speed?

If you're getting steering wheel movement at low speed it's usually caused by either a bent wheel, a damaged or bent CV joint/half shaft or a bad tire.

If it's the tire or the CV joint you need to take care of it immediately before something bad happens. If it's a bent wheel, that's an annoyance that only you can evaluate, but it's not as much of a safety issue as the other two problems.