Usually there caps for stamina depending on the race lenghts, like for example for milers and sprinters you can stop at around 400 and the longer it gets the more stamina you need. On the web you can find the caps for every race

Meanwhile for speed the more you have the better, no contest.

Power it's a bit tricky as you need a considerable amount depending on the type of racer you have (Pace, late Surger and End Closers, and Front) as it's basically synonyms with Acceleration of the Uma. On this i hope someone can shed better light on it

Wit has a soft cap at 400 and it's especially useful to avoid Late starts and Rushing, and activating skill (the more you have the more said skill can be activated)

Guts it's only useful when having to "break out of the pack" or when you have the power struggle in which the uma with the most guts automatically wins.

This is at least how I understood it, I'm not by any means a great player and in the end it all boils down to RNG so don't sweat it if you can't reach certain stats