Tip tanks are a mixed blessing with regard to wing bending moments. Full they can give bending relief (although inertia in turbulence can undo this) as, for the same amount of fuel carried, less of it may be in the fuselage (the Canberra is a case in point). However, empty they increase wing bending stress as they move the centre of pressure outwards (reduction of induced drag and increase of lift at the tips because of reduced spillage from lower surface to upper surface). Canberra 'g' limits were +3g (or was it +3.5g?) with tips and +4.5g without.Their effect on B:A ratio with regard to spinning is very significant. Full tips will make the spin very stable, so much so that recovery may become impossible. Don't spin a JP or Strikemaster with fuel in the tips!