Tuning for stats is totally dependent on which bot, parts, and the stats on those parts. You have to do some simple math to maximize the number of slots you have with the amount of cost you have. Then you have to pick which tune ups to use be it high, great, alpha, gamma, etc...
Only rules you really have to follow is not to have 0 on any of your stats. There are exceptions of course which is dependent on your play style.
Getting 0 on either STR or TEC results on a 3 second wonder bit usage. Only get it to 0 if your play style doesn't revolve around wonder bits.
Getting either WLK or FLY to 0 is never advisable with any play style. At least leave both at 1 if you choose to do so. Mobility is Key in this game! Try to tune up without sacrificing a single point on these two stats.
TGH is iffy really. Of course don't get it to 0 but also don't tune it up if you're not going to tune it up to the max. People's opinion on this is different but mine is if it's not 40 it's not worth it.
As for weapon tune ups, my opinion on this is get the tune up that gives the most. I.E. I would would rather get Great Grappler rather than just regular grappler. There's always the exception that if using the weaker one lets you fill all the weapon's slots because of cost restrictions, then it's better than nothing.
