If your using the ring collar with the lens, the everything is balanced from there and vertical or horizontal won't matter. Another thought is to get an body specific "L" bracket from kirkphoto.com.
Check out this link from Really Right Stuff regarding a tech tip for using monopods. I have this with some larger lenses (I'm using the lens ring collar to switch to vertical and such) on beefy Gitzo and Manfrotto aluminum/carbon fiber monopods.
1DS Mark II, 20D's, Kirk, Wimberly, RRS, Gitzo's, Lowe Pro's, Kinesis Photogear, Pelican cases, Lee filters, Hensel monolights, 15 2.8, Sigma 20 1.8 EX DG, 35 1.4 L,
50 1.4, 60 EF-S Macro, 85 1.8, 100 2.8 Macro, 135 2.0L, EF-S 10-22,16-35 2.8 Lv2, 24-70 2.8L, 70-200 2.8 IS L, 200 1.8 L, 300 2.8 IS L, Pentax 6x7, Photoflex and Chimera.