The only negative in feeding is that it's wasting your MGP/money. If you have a lot of MGP/money to get the food then it doesn't matter.

Only your chocobo's stars are passed on when breeding, so any stat gains due to feeding are not passed on. That's why it's not "worth" it until pedigree 9 (max pedigree). Nevertheless, feeding your chocobo increases your chance of winning, speeding up levelling so you can breed sooner. Plus, a pedigree 5-8 chocobo racing without food would be extremely frustrating as the NPCs will be tough to beat.

Also, be careful on confusing ranks with pedigree. Ranks and pedigree are completely different things. I think most guides are saying for you not to feed until Pedigree 9, not rank 10.

I'd recommend feeding grade 1-2 foods which you can buy and craft until pedigree 9. If you are rich on MGP, a few grade 3 foods around rank 8-9 would help make levelling races faster. Feed only grade 3 foods to your final "perfect" pedigree 9 chocobo that you want to keep.