ive taken 4 walkons a lot of times and you don't really get a big hit on the practice plan. maybe instead of +8 a guy is +7, but its not like you are going to see improvement cut in half, or anything like it.
my rule of thumb is this - totally ignore the practice hit, and make your decision that way. as was mentioned, its much better to have a walkon play (or usually, not play, at least not more than a couple end of game blowout minutes) in place of a ****** freshman for 1 season - than to sign a guy who cant compete and be stuck with him for 4. now, at some point, a lot of walkons will hurt your team - if you take 5, you have to run with 7 men, and that is going to hurt your season. still, its often better to take 5 than to load up with guys who suck. but really, you can take 2 with ZERO concern, 3 with VERY LITTLE impact, and even 4 is not that big of a deal. the bigger deal is often to class sizes and such, than it is to the performance of your team, that season. maybe your s16 type team only makes the 2nd round. but you can get over that - what you cant get over is getting 2 crappy FR so you can still make that s16 and then sucking for 3 the next 3 seasons.