The primary function of a desk grommet is to cover up the sharp edges on wood, metal, and glass desk cutouts, so that computer and phone wires can travel through them without being abraded or cut. If protection is the reason why you're looking to purchase a grommet (or several), then you can opt for pretty much any style you want, since as we just stated that's pretty much their main deal.
On the other hand, if you'd like some help routing cables around your desktop once they've transitioned through it, your best bet would be to narrow your choices down to Flexible or SoftTop grommets. They'll help you protect your cables, but also let you have some control in where you choose to direct them.
Flexible grommets are covered in a soft, rubber-like, self-healing material that has been radially slit, or cut like a pie (or pizza!), to allow cables to pass through it. Since these slits are flexible and radiate outward in all directions, they let you decide which direction you want each cord to go in, and then they gently grip onto the cables to keep them on the right track. Any unused routing slits lay flat and stay together for a neat appearance.
Much like Flexible Grommets, SoftTop grommets also use a cleverly perforated membrane-like top to route cables. Instead of simple pie-cut slits, this grommet features a unique, stylish cutout that will remind you of the Cingular logo (remember Cingular...no? Okay, just Google it). The three curvy arms of this opening allow cords to be directed in any one of three directions and add a little hip and modern detailing to otherwise blah work surfaces