The slide rails are the structural support that was engineered to hold the slide when both in and out. Never use supports in the event of settling as others have mentioned.
If the slide appears to be sagging or the like, chances are that it may either need an adjustment (not something for the average person to do) or you have a structural issue that needs to be repaired/replaced.
If the slide is properly adjusted, it shouldn't have much bounce when fully out.
2009 Salem PT 403FB
2005 Ford XLT Supercrew