if the scope does not have turrets it really isn't that big of a deal, as long as it looks decently straight you are likely good to go, however if you intend to use turrets to dial up your range it gets very tricky to get it right, IMO to start with you gotta have one of these
http://www.brownells.com/aspx/NS/store/P...LE%20INSTRUMENT
its about the only way I know of that you can have your gun level to start out with in a gun vise, which is most critical, I also use it to attach a leveling device to the scope, which I flip up when making a shot, I secure the above device with the gun in a vice, then I look through the scope, the EXD device has a slot that allows you to see through it, then I level the cross hairs against my neighbors front door behind my house, its about 120 yards away, (they don't know I do that LOL) I normally spend about 45 mins fiddeling with everything to get it right, I also don't trust the small levels that you can set on top of the scope turret, that also implies the crosshairs are aligned to the turret, but what if they aren't?? you also should check this at the range, by putting a target up with an upside down T on it that is of course level and perpendicular, shoot a group with the intersection of the t on the bottom, then dial up the scope as far as you intend to shoot, then shoot another group, if the groups are dead center right on the line, you are good, if they are to one side of the line or the other, you need to rotate the scope slightly, a 1 degree cant will cause you to not only miss but also hit low.