Your cue stick should be as level as possible. If you are jacked up, and you hit the cue ball off center (left or right), the cue ball will masse. This masse may be very slight, but it will happen. Players who are jacked up all the time have a hard time with long shots becuase that is where the slight masse will affect it the most. Of course if you have shot jacked up your whole life, you may be so used to it that you can still play well. If you are not one of those players, I would recommend that you keep a level cue.
There are some exceptions. If you are shooting off the rail, you need to elevate a little. Also, if you need extreme draw, you need to elevate a little (very slightly). I'm not talking about drawing the cue ball back 1 table length. I'm talking about drawing the cue ball back 2 or more lengths. Maybe shooting the cue ball 1 table length to the OB and then drawing back a full table, off the short rail, and up a little. A slight elevation will help get this kind of action, but you will need to have an almost perfect stroke or you may miscue.
Allison certainly plays great. Her mechanics were learned in Europe. They position their legs differently, and they get way down low so that their chin almost touches (or in some cases does touch - see Steve Davis) the cue. That is just the way they are taught over there. One way is not better than the other in my opinion. You just do what feels natural. Personally, I can't get down that low and aim consistently. My body needs to be a little higher up. On some shots (extreme stroke shots), I stand even higher than on a normal shot.
Good Luck
Andy Segal