Unless you have vertical movement 100% under control at not only the four corners but the mid-length of the frame, I would never use a jack or support under a slideout. Take a look under your TT while someone walks in and out of the stairs heavily. You may be surprised how much movement there is. If the frame is not restrained enough and you prevent the slideout from moving vertically, the movement between the slideout and TT body will end up damaging something. That's my take on it but I'm sure there's many who say there's no problem. If you have manual scissor jacks you'll be better though compared to electric stab. jacks which can move around a lot.
Perhaps if you do more to minimize bounce and movement of the frame you will find the slideout is more comfortable. You might consider bracing like the BAL locking arm stabilizers (or equivalent). Have you tried some bottle jacks under the frame just ahead of the wheels?
Gil & Deb & Dougal the Springer Spaniel
2014 KZ Spree 262RKS & Ford F250 supercab V10 4x4 LB