Personally, that would be on an as needed basis. If you are driving in heavy rain, but getting good traction, leave the 4x4 off. Driving through flooded roads, you get the "de de de" award.
Gavin, his is an '05..isnt the 4x4 system different from '07 and '08?
Yeah, a lot different.
The 4WD system on the older tracs should not be engaged unless there is provision for the tires to slip a bit due to the lack of a center differential. I would not use the 4WD on wet pavement (in the rain), or even off-road if it were not wet and/or very loose. If the tires cannot slip, it puts one hell of a strain on the drivetrain.