I found this to have minimal effect in zone coverage in M16. Haven't tried it much yet with M17 but the pros & cons of zone shading will be the same as I've listed below if it works better for zones in M17.
Pro/cons for shading in Man coverage:
Under - use this if you think the pass will be a short or medium throw. This leaves you vulnerable to an over the top deep ball.
Over- Use this if you think the pass will be deep to keep your defender over the top of the WRs route. (3rd & 15+, hail marry plays, etc)
This leaves your DB vulnerable to underneath short/medium throws.
Inside- If your opponent is running a lot on ins/slants/etc where the WR breaks to the inside use this. This keeps your DB to the inside of the WR but leaves you vulnerable to outside routes, flags, etc.
Outside- The opposite of inside, this keeps your DB to the outside of the WR towards the sidelines.
Basically you don't want to shade the same way unless you are playing someone who only looks to throw the same type routes over & over or looks for the same thing every 3rd down. Each one has its pros & cons. It is also subject to your DBs ratings, so if your DB doesn't have good cover attributes or PRC he will struggle defending any route except fly/go routes where the WR makes no cut or route other than running in straight line.
Personally here's how I use them.
Under- 3rd & 12 or less & my opponent runs a west coast style offense with short & quick passes.
Over- 3rd & 12 or more or hail marry plays where I assume my opponent is trying to hit a go route. (think 4 verts)
Inside- Only when I play someone who will continue to only try hit slant routes for his 1st downs. You can get burned outside but trying it will let you know whether they are going to keep trying to force those throws inside.
Outside- I probably use this the least. Sometimes in drives where my opponent is trying to get out of bounds with time running down. I find putting your DB close by in a purple zone (curl to flat) works best to take away the flag route better than just only shading.