i'll occasionally set a player to -1 if they are at 50 per, especially if they suck at the free throw line. if they are in the mid 60s or better ill set them to -1 or 0 and just see how it goes. sometimes they hit nothing and i'll ease them back, but if it goes well sometimes i'll let them shoot a few more...again especially if they are poor free throw shooters.
i've definitely had guys under 70 who were effective, even at D1 if they have some athleticism, speed, or other good skills to compliment. Just had a D1 guard finish 60/143 from out deep for his career as a 68per. but in general i tend to agree that guys under 70 really shouldn't be jacking up too many 3's as a rule, though there are situations where maybe it's worth the risk depending on the makeup of your team, what kind of defense youre facing, and what the history of the individual player is