In my experience, which is fairly extensive with potential, once the maxed-out email is received, lowering practice minutes to 0 will cause the player to lose points VERY slowly, no more than 2 points in the entire season, many times only 1, and some times none. I have never seen an exception, but others may have. I can only speak of my experience.
Receiving the maxed out email signals that the player could gain a maximum of 1 or 2 more points to reach full potential, so having a player get such an email allows you to move minutes out of the core skills and into others, holding those core skills very near the top, while starting significant growth in non-cores. For instance, if the player is a C with maxed 95 REB and LP early in his Jr year, those minutes can be moved to BH and PE. Let's say the C has 5 ratings with potential of 20, and let's say the LP/REB minutes totaled 40. Then REB/LP can be dropped to 0, and BH/PE can be put at 20 each, allowing the C to work toward 20 (instead of 5) by the end of his Sr year, while pretty much holding constant in REB/LP. (Or some of those 40 can also be spent to increase PA, to reach that max quicker, while also putting some into BH/PE, etc.) So instead of trying to get the last point or 2 in LP/REB (to get to 96 or 97), while remaining at 5 or less in BH/PE, the C can become a 93-95 in REB/LP and a 20 in BH/PE. Some may prefer to absolutely max those cores while leaving his non-cores as is, but I prefer to get players as well-rounded as possible, with only minimal (if any) cost to the cores. JMO.