With B738 when tnking is recommended we flight plan a maximum of 6 tons extra fuel for flights that are longer than 1hour due to wing icing problems and up to max LDW -300kgs
for less than 1 hour EET, provided the next sector is not in any way performance i.e TOW-LDW limited.
Long range a/c on short haul routes will often tanker enough fuel for both sectors, and when conditions permit will tanker on long haul routes as well, not so sure about the implications for wear and tear on the engines and airframe when the aircraft are routinely operating at at close to max wgt, I guess this muddies the waters further......