If the wear bars are showing, not tread down to the wear bars, but just if the wear bars are showing at all, you should seriously consider replacing the tires soon. If you wait until the tread is down to the wear bars, it's far too late. When the tread is down to the wear bars, you have 2/32 or less of tread left. The wear bars are 2/32 of an inch deep.
Steer tires must have at least 4/32 of tread left (the top of Washington's head on a quarter).
Drive tires must have at least 2/32 of tread left (the top of Lincoln's head on a penny).
These measurements are for the center grooves of the tire, taken at least 15 inches apart, as well as for the inner and outer grooves taken a various spots on the tire. The shallowest measurement of tread anywhere on the tire is considered the tread dept of the tire.
Tread depths of 4/32 or less cannot negotiate wet roads. Water cannot be compressed, so there must be enough room between the tread groves to allow the water to escape. At 4/32 or less your vehicle simply has no choice but to float on top of the water, as there are no grooves with which to move the water out of the way.
For snow traction, you need at least 6/32 of tread left (the top of the Lincoln Memorial on a penny).
The original Michelin's found on most US Sprinters (LT 225 / 75R - 16) are 80,000 mile tires. At a ballpark figure of $800 for a set of four, it will cost you right at one penny per mile for those tires, at 80,000 miles. If you gain 25% or so with proper inflation (say, with nitrogen), that's 100,000, and is a cost of $.008 per mile. That's a $40 savings over replacing them at 80,000 miles.
When you get up in that range of miles, the difference between 4/32 and 2/32 is generally going to be between 10,000 and 20,000 additional miles. Going with the higher of the two at 20,000 additional over the 100,000, the cost savings of squeezing the last 20,000 miles down to the very real dangerous 2/32 wear bar mark is $0.0013 per mile, or about $26.00.
It's not a contest. Bluntly put, $26 is simply not enough to risk the lives of myself and others. When my tread depth hits 4/32 I'll replace all four tires (doing that today, actually), rather than trying to squeeze out another $13 on a pair of drives.
Slow and steady, even in expediting, wins the race - Aesop