Only loosen the brake pads enough that you can still get adequate braking performance. That's really not an ideal solution though.

Are we talking about a slight, say, <1 millimeter wobble here? If that's the case, play around with truing it some more and see if that takes care of it. Remember to make small changes at each adjustment.

If it's a huge movement, your rim may be damaged. It may also be a spoke issue as mentioned above. An LBS can assess whether one of those is the case.

Either way, play around with truing it for a bit and see if you can make it better. If that fails, take it in and have a mechanic true it. It's not like it costs you any more if you screw it up further before taking it in