This is how it works:
If you inform of redshirt and sign the guy, he WILL accept the redshirt with no complaints 100 percent of the time.
If you don't inform, there are a number of factors which determine if he'll accept it, including (from my understanding, anyway) a bit of randomness. It depends upon the talent on your team and the talent on your team at his position among other things.
Generally speaking in my experience, any non-freshman is much harder to redshirt (I've never done it without informing first, but I'm told others have). Also, more talented players overall and pull-downs seem to be more difficult.
Here's my rule of thumb: If I know I want to redshirt a particular guy, I will inform him UNLESS 1. I'm in a battle for him; or 2. I don't have much recruiting cash left (or won't with other battles I"m in). Once you inform, you've got it made as long as you sign him.
If you take a chance and he doesn't accept it without a work ethic loss, determine if you're okay with that amount of work ethic being lost (forever except for gaining it back the same way he otherwise would). Sometimes they only lose a few points, sometimes they go ballistic and lose a bunch or all of it.
If you don't like the work ethic loss, take the shirt back off of him. Put it on someone else if you want to give them a shot. Or just don't use it.
BOTTOM LINE: It never hurts to try a redshirt, but if you want to make sure, just inform during recruiting.