You can't randomly space an external cup crankset like Sram/Truvativ or Shimano. The bottom bracket/crank combo is designed for a certain width bb shell and/or number of spacers. In the case of Omniums, it is designed for 68mm shells with no spacers. By adding spacers, you're preventing the non-drive side crank from bottoming out on its stops against the bearing and not seating fully. This is bad.

Re: spacers, usually mtb cranksets come with a couple of spacers for use w/ 68mm bb shells when the cranks are also compatible with 73mm shells. For example, Shimano hollowtech 2 mtb cranks use 2 spacers on the drive side and one on the non-drive side when used w/ 68mm shells. When used with 73mm shells, you only use one spacer on the drive side, and none on the drive side. You can switch them around to move your chainline left or right, but you need to use the recommended amount of spacers for your given crank/frame combination. Read your bb and crank manual.

IIRC, Sram GXP cranks, for both road (Force, Red, Apex, Rival) and track (Omnium) are designed to use 68mm English or 70mm Italian bb shells (using the correct BB) and no spacers.