Currently my single speed mountain bike has a Fat Frank in front (26X2.35) and a Fat Boy (26X1.25) in the rear. The only problem with my set-up is now I have a slopping top tube.

As most fixed gear riders know, it's no big deal to skid a rear wheel, or to even have it wash out a little. If you front tire starts to skid, bad things happen very quickly. A wider tire up front minimizes the risk of this by providing more traction due to the wider contact patch.