Do wheels need rebalancing - peteH
Do wheels need to be rebalanced, unless the tyres have been removed from the rims.

Can a tyre get out of balance over age?
Do wheels need rebalancing - Dynamic Dave
Do wheels need to be rebalanced,

Yes. You only have to hit a pothole, or mount a kerb in anger to knock out the balancing.
Do wheels need rebalancing - Cyd
Most tyre manufacturers recommend rebalancing at about half worn.
Do wheels need rebalancing - AlanGowdy
.... but beware. Very few establishments, large or small, seem capable of properly rebalancing. That is to say, leaving you with a car that is free of self-induced steering vibration at all speeds.
Do wheels need rebalancing - peteH
mmh. Ive heard this before.

is wheel balancing really that unreliable - are you likely to be worse off that before?

I understand that balancing can be done "on car" - how does this work? Who does it? How much does it cost? Is it worth it? Does it affect abs? (rotating wheels off the ground?)

Value my car