It is important to have the scope level with the action and it's also very important to avoid canting the rifle when you shoot. There are several devices on the market designed to mount the scope level with the action, but I have never used any of those devices. I use the following procedure.
First I make a target backer that features a vertical line made from a 3/4 inch wide piece of black masking tape. I place the target 100 yards form my bench and make the tape plumb with a 4 foot level. I then place the rifle in a cradle or lead sled. With rings in place and lapped, I place a level on the flat part of the reciever or on a flat portion of the mount. I level the rifle and tighten in place. I then mount the scope loosely and check the rifle once again for level by placing a precision parallel on a flat part of reciever or mount. With rifle level, I aim at the black tape. I then rotate the scope and check it with a small level placed on the top turret. I double check by making sure the vertical crosshair follows the black tape. Rifle level, scope level, vertical crosshair plumb. Tighten the screws in a criss-cross pattern, constantly checking to maintain proper position. Now is a good time to mount a Scoplevel on the tube if so inclined.
Now, sight the rifle in. Then place the target low on the backer with the vertical tape extending a couple of feet above the target. Move the vertical adjustment in the scope up a bunch, like several turns and shoot another group. Use the tape to make sure you don't cant the rifle. The second group should hit a couple of feet high (depending on how many moa of adjustment you have) and the group should be centered on the tape.