When you get your wheels balanced, you may not be surprised to learn that
most tyre shops are pretty rough with your wheels, and not too careful
about one of the main reasons for your visit - balancing!
When they fit new tyres or valves, try to find a shop who will let you check the wheel rim for corrosion - or ask them to check for you. If your tyres have been deflating slowly then suspect this as the cause. Old alloy starts to tarnish, and when in the tyre bead area it can cause air leaks as the tarnish lifts the rubber and gaps appear as the tarnish starts to drop off/powder. If any is found rub off with some emery paper (80 grit will do) and then ask the tyre fitter to apply the tyre soap on the area so an airtight seal is more likely to last.
your wheels are balanced, you will almost always find the fitter puts
on a few weights. This is ok as the wheel may require more than one a
side, but you may be surprised to learn that most machines have a "fine"
setting...which indicates the weights that are needed to the nearest gram.
The normal weight test goes to the nearest 5 grams - so if 3g is needed
in one particular point the machine will say 5g. Although this will work,
and the slight difference won't be obvious, it is something that if done
properly could make your tyres last a little longer, and also fuel economy
could be improved.
In addition, I have tried rebalancing a part worn tyre that I balanced to zero grams (i.e. exactly balanced according to the machine). Interesting to note that the tyre wear had caused the wheel balance to go out again. The weights had not fallen off and the wheel had not been damaged. This shows that balancing is perhaps more important if you want to get your car set up as good as you can for economy, performance & comfort.