By the time your a few years into your dynasty, your goal should be to only be redshirting freshmen. If you're managing your team and your recruiting properly, this shouldn't be too hard.
However, just because a player is a freshmen, doesn't necessarily mean he needs to be redshirted.
Ultimately, redshirting is about depth and future depth. Keep in mind that at any point in the season, you can remove a player's redshirt and put him back on the depth chart, so you don't need to plan too far ahead as far as injuries go. You just need to pay attention when there is a serious injury that you fix the depth chart to get the redshirt player back in the mix.
So, is it smart to redshirt all freshmen with low OVRs? Generally speaking, yeah, sure. When I first start with a team, I tend to redshirt anyone who isn't making the 2-deep depth chart. I'm a run-heavy team though, so at RB, I keep 4 active. In any given game, I'll have 3 RBs with at least five carries, so the 4th guy really is only 2-deep I guess. I then always have a 5th guy redshirting (and sometimes as many as 6 RBs on the roster).
You do have to be careful with redshirting. Upperclassmen who should be seeing the field tend to transfer for lack of playing time. Underclassmen can transfer too, whether they highly rated early playing time in recruiting, but especially if you promised them something involving playing time (freshmen AA, solid playing time).
But as I said, it's about future depth as much as anything.
For example, if I have a redshirt senior QB rated 90 ovr, a sophomore QB rated 83, and a redshirt sophomore QB rated 82, then I will redshirt the sophomore rated 83 as he is eligible and will be hear a year longer than the one who has already redshirted, and it's not much of a drop-off to the third guy, even though the guy I'm redshirting is on the two-deep.
HOWEVER, if I have, for example, a redshirt senior QB rated 90, a junior QB rated 83, and a redshirt sophomore rated 82? I tend to
not redshirt the junior simply because I don't want two of my three quarterbacks graduating in the same year. At any given position, I try to stagger my players leaving, so I can recruit more efficiently. It's easiest to go after one player per position, I find.