It all depends on the length of shooting you plan on doing as well as what your purpose of the rifle is. If you plan on not shooting over 40-50 yards, then you don't need any adjustable riser. Just zero in to the range you want to shoot and you're good.
If you're like me and want to do what Trinity speaks of, then an adjustable riser is a must. There are different kinds out there, like the Tiberius one and the Killjoy APR (I, myself, like using my HHA Optimizer. ^_^). You just have to find which one fits your shooting style. The Tiberius and Killjoy risers sit high on the marker, and are useful for large changes in elevation. The HHA has a lower profile and is for finer adjustments, but does not adjust to the extent that the other two do. Some have combined two of them, like putting a Killjoy on top of a HHA to have both large and small adjustment options. I use just the HHA and zero in at 40 yards. The full adjustment of the HHA gets me out to about 120 yards, which is plenty for me seeing as I do all of my shooting from 50-100 yards out.
I guess when you look at it, its all about what you want to do on the field. I know of people who use SAR 12s with no riser at all, and they are good shots with them. You just gotta find out what is best for you, and make your marker your own. There is a wealth of knowledge about sniping over on the ACES forums as well, so you can head over there and browse around if you would like more information. Happy Hunting! ^_^
"Sniping's a good job, mate! It's challenging work, out-of-doors... I guarantee you'll not go hungry, 'cause at the end of the day, as long as there are two people left on the planet, someone is gonna want someone dead."