Our experience has been that they are accurate...so more generous than other parks we have been to (Kings Island). It is pretty consistent too...for our last trip, we didn't think our 3yo had a shot at 40 (wasn't at our local amusement park or at home), but was close, so with the right shoes he was tall enough and was always allowed to ride (pleasant surprise for us). Once the CM even helped with his posture...suggesting that he raise his arms up above his head (feet were still on the ground). I suggested the same at Kings Island, and we were told we had to keep our arms down.
The CM won't bend the rules (look the other way), but will do everything they can to see that a kid has a chance to ride safely.
I wouldn't sweat it, but I would pack tennis shoes vs. flip flops.