Agreed. "Topping up" a script is a fast-track to turning a dependency into a full-blown addiction I'd say. If you can learn to deal with the anxiety from skipping a dose or falling short that is a far, far better way of going about things. If weed helps I wouldn't personally consider that a huge problem unless it becomes another crutch and ends up causing more harm than good cos you are in a situation where you need both weed and your script to get by. I have always found benzos to be highly effective for anxiety but only when I use them very specifically for that and don't take them on days where the anxiety is not so much of a problem. Taken daily you inevitably become tolerant and start to either need to go back to the doc hoping to get your pill count raised or to start looking for alternatives like buying from vendors or using other substances to "bridge the gap" as you say.
Obviously I don't know your situation but if there are days where anxiety is not such a problem (days where you're basically sitting around not having to get into situations which may trigger anxiety attacks, for example) then I'd suggest skipping your dose on those days. I also found that simply having benzos on me meant I didn't always actually need to take them - simply knowing there was a solution at immediate hand if I really needed it meant I needed it less often. If it's possible to have your dose on you but only take it if you feel you really do need it rather than taking it in case you need it that will also help keep tolerance in check and perhaps even help to find coping strategies other than scripted ones over time.