Finding a good balance is the hard part. What works for one car won't necessarily work with another car.
It can also depend on what tyres you're using and how much power the car has, aswell as other settings (like ride height, roll bars, dampers, LSD) that can also affect rear stability and grip.
You're 100% right though about finding a balance, not only with the different options you have in settings, but the results of these too.
Balancing tyre wear, lap times and overall race time is also important.
The main reason people don't overdo toe is because if you were to turn tyre wear on, you'd find the tyres would wear faster with higher amounts of toe, because you're dragging them down the road at an angle, which also slows you down a bit.
It is an effective way of changing the handling of the car though, there's no debate about that.
Yes, if 'abused' a high rear toe figure can murder your tyres, people need to be careful, it's not a 'magic cure'.
If used correctly it can work very well.
Spoiler robs you of to much PP on a PP restricted car, I would much rather use a little rear toe, but only as last resort. I usually never use more than 0.08 in either direction
Yes, aero is a very high use of PP, not 100% worth it in my opinion, some cars yes, but not all cars.
Some cars I race I've found that adjusting the suspension and LSD will give you more rear grip and stability than a spoiler anyway - without affecting lap times or tyre wear.
In real life spoilers do nothing below a set speed, the faster you go the more downforce they'll generate - whether this is accurately reflected in GT5 is another thing though.
If you adjust your suspension and LSD for rear grip and stability, this will work at all speeds, not just high speed.
A spoiler will do nothing for grip and stability at low speed 2nd gear corners / hairpins, LSD and suspension settings, will, in all corners.
