Even our Ops manual says the same but in practical flying, with bare minimum reserve fuel, it kinda becomes a trade off between deviation and having reserve fuel left for any delay at Destination.
I wouldn't be comfortable flying for a company where the fuel was so tight that diverting around a cell by 20nm effected my options to that extent. Why not just put on an extra 300kg if there is a chance of TS enroute?
If you are that tight at destination how will you cope with a flap checklist on approach? I'm interested to know if there are companies where putting on a bit of gas for this would be questioned.