Fronts for me, pinched tubes (high country ride 2006 had 3 in one day)even ultra heavy duty tubes can be pinched when you drop pressures searching for traction. Also damaged rims, dented front rims when using tubes and low pressure was a problem I solved when I switched to mousses.
Think I have only had two rear flats that I can remember in my adult life, once because of hitting a hole and broke two spokes, I stopped and saw the broken spokes, turned for home and soon had a flat from the spokes not be cable tied and piecing the tube. The other was from to low a pressure at Louee because of the rocks and 14psi
TB