This is true. Most of our bikes come standard with lightweight alloy valve stems as well, which negates that imbalance a little.
The lighter forged/machined wheels are usually pretty good in terms of weight balance, but the cast wheels can be out a bit in the way Shazaam describes.
I had a rear wheel on a '90s SS which had a heavy spot away from the valve. If you find and mark that point and move the tyre spot there you can save on wheel weights.
Of course this imbalance is often a function of the tyre manufacture, and some makers are worse in this respect than others. Tyres are not all created equal in terms of both balance and running true..
But I don't like having as much as 50g in weights on there, particularly after spending so much effort and $$ on lightening every fucking thing on the wheels..
