As a very experienced MK player, I rarely prioritise grip (except for 200cc on some courses where you need control).
Grip (or Traction in MK8 (USA), or Stability in MK7 (Hidden)), determines the vehicle's ability to remain stable (e.g. flat, not slipping/sliding) when on slippery terrain, or when landing on a new surface, especially when turning. It is NOT the ability to take sharp turns (Handling) or to drift tightly. The differences between low-grip and high-grip karts are very subtle, but can be noticed more in 200cc:
Karts with a low grip slip more on certain surfaces (like ice, mud), and seem a bit less responsive to some turns. Also, it's harder to drift left and right successively, since the kart won't turn well during mini-turbo boosts. But, they can still drift/turn tightly (dependent on handling/drift stats). On the other hand, karts with high grip slip less (but still slip), and seem to respond well when turning and landing.
Overall, unless you're driving in 200cc on Mount Wario, or similar, grip should not be a top priority. Maybe keep it in the middle, but focus on other stats.
