Yes you will get disc creep with extended sitting. If you don't have stenosis, then the most important thing is to keep the lumbar curve in place by supporting it. OTOH, if it is true sciatica and not caused by the disc, the issue is the compression at the piriformis (butt) muscle. In which case, foot positioning is very important. If you have stenosis--moderate to severe, nothing will really help except maybe leaning ever so slightly forward.