Laws vary. It probably is not legal on 1, 2 or 3 counts:
Exhaust must exit between 0-2' or more than 11', some states.
You can't block the forward view of the driver.
You must have some kind of muffler.
Realistically, unless you are doing something dumb, you won't get a ticket. But the exhaust slime is a good point. I ran a cut-out at one time, and had to stop because all the greasy exh residue was all over everything. Probably wouldn't be so bad on the top of the truck though, cause then it only looks like shiit. When it's under the truck, it makes it harder to work on stuff since you spend more time cleaning than normal.