In essence a 20 MOA rail is a wedge. A 10 is a lesser wedge and 30 a more severe wedge and of course a no cant rail is ideally parallel to the receiver.
What a cant rail gives you is more elevation adjustment on your scope to allow you to adjust for longer range shots, if, the scope has limited elevation adjustment in the first place and adjustment for elevation has to do with physical size of the tube (why most LR scopes are 30mm main tubes or larger). If I remember correctly, a 20MOA cant rail is 0.014" difference between the extreme ends of the mounting pads (Fore and Aft).
When you buy a long range optic it's always a good idea to check the specifications and especially look at total MOA or MRAD adjustment, the larger the number, the wider range the scope has.
The Vipers are middle of the road so LR necessitates a cant rail. I have a couple. One on a 308 with no cant, one on a 338 with a 20MOA cant. Some Vipers exhibit a bad tendency and that is, they will show a crescent ring in the ocular when adjusted to the lower limits of elevation, neither of mine do, but some do. The crescent you see is actually the lower radius of the tube becoming visible in the ocular.
Just mounting a scope is less than half the process.....
Don't forget to align the rings parallel to the receiver... align the scope itself parallel to the receiver (to remove any radial cant in the crosshairs) and torque the mounts to manufacturers specification and lap the rings true with a suitable lapping bar and lapping compound. I use a machinist level to remove crosshair cant but any small bubble level will work. With the rifle in a gun cradle like a Tipton, level the gun with the bubble level crossways on the top of the rail or feceiver if flat, then place the scope and place the level crossways on the elevation turret or cap and level the scope to the firearm. That removes crosshair cant and aligns the crosshairs parallel to the receiver, very necessary with long range shots. Mounting a scope level (like the Holland) on the scope, keeps it level when shooting LR. You want the scope and rifle level at all times when shooting, A rifle canted to the side will change the POI of the bullet at extreme distance. Additionally, you might want to add an angle cosine indicator
for shooting up and downhill. BTW Len Backus offers both on web store on this site.......
When you set the scope in the rings and adjust for proper eye relief (eye box), be sure to torque the rings themselves to the proper torque specification and if they are multiple fastener, progressively torque the mounting screws.
Finally, I use a dab of wicking grade threadlocker on the torqued mounting fasteners to insure they don't loosen,
Wheeler Engineering makes some good scope mounting tools including lapping and alignment bars and torque setting screwdrivers. I made my own lapping and alignment bars and actually any competent machine shop can make a set from suitable bar stock inexpensively.
Improper mounting causes the 'scope ring marks' you see referred to when someone is selling a used scope. The rings were out of alignment in relationship to the tube causing a mark on the tube when the rings were torqued down. Severe misalignment can actually cause failure of ther scope itself by twisting the tube. That causes the erector mechanism to cease to function and the scope is junk.
All rings (with the exception of the Burris insert rings) need to be lapped and even the Burris rings need alignment.
Finally, keep in mind you want the maximum ring spread spacing in relationship to the tube itself within the constraints of eye relief and rail/receiver placement as possible.
See, I just complicated the process.......