as a category approaches maxing out, it takes significant practice time to improve. Likewise it appears to also drop slowly as well.
Is it safe to put 0 minutes into a category and not see it drop? No, a rating will decrease with 0 practice minutes. But it will drop very slowly if already in red. Maybe to a degree that you wouldn't see a point lost in the category. ie.. it may drop around 1/2 point. So if the decimal rating is 10.3 and it drops to 9.8, the whole number rating stays at 10. If it begins at 9.8 and drops to 9.3, then you will see the category drop from a 10 rating to a 9.
If you have another category that still shows room for improvement, moving the practice minutes to that category will probably gain more than the red rating will lose.