Regarding car weight distribution

For racing diecast cars, I've seen some people say that you want your car's weight to be predominantly in the rear, and some say it should be in the front. Where do you think someone should balance their car for a wide track?
I've noticed some cars can really struggle to stay on an open track it they are super light in the front (or top heavy), but also it's common knowledge that in a straight-line race cars (like pinewood derby cars) having your weight closer to the rear axle gives you more speed.
Also, any crafty tips on how to attach tungsten/lead weights to a diecast car as low as possible? Do you cut holes in your models and glue tungsten weights in? Do you melt lead weights and get creative with placement? How do you cast them if you melt them?
Discussion
I use 158°F diecast (woods metal) to make the weights.
It can be poured directly into plastic, and not melt it
It's density is lower than most metals used for weight,
but can completely fill tiny areas and stay put.
I add to the nose, and tail first, then then balance with weight in the middle
dr
- Bismuth... interesting choice. Not quite as dense as lead but maybe less toxic and easier to form. — Stoopid_Fish_Racing
- Innovative in using solder to "fill-in" small spaces, also interesting to learn it does not melt plastic. — dcdautos
Be sure to browse the collection of topics about weight too, lots of good tips and discussions in those lists.
For open track, middle to slightly forward is often better but for drag, further back gives you more potential energy.
Having hosted a few customs, i see all kinds of weight adding techniques. The lowest have been when the builder cuts a section out of the chassis and replaces it with the tungsten block. Otherwise people have usually glued it into the centre of the car, under the interior, with anything from no more nails to modelling glue.