You can burn contingency when you dispatch so in my view no big issue.
Taxi fuel in my company is based on a statistical analysis airfield by airfield but the unexpected can and does happen. If you are aware there is an issue prior to loading your fuel then load more. If the problem is unexpected then you can hardly have been expected to forsee delays above and beyond what is normaly expected and accounted for on your fuel plan.
If your plan assumes longest departure and arrival then happy days if you are useing a shorter departure or arrival as you can correct your fuel required accordingly and maybe find a couple of hundred extra kgs that way. Same for ZFW verses the plan etc. You may know from experience that you alays get a large shortcut and end up 300 kgs up on that route in any case.
At the end of the day its a judgement call and you have to take the relevant factors on the day into account and make your decision. If the situation arises were you are burning into contingency then you will have to keep a very critical eye on things. Conserve fuel shut an engine or two down. Should you feel you may need to return to stand then it would indeed be appropriate to exercise CRM in its broadest sense and prempt the problem by chatting to your company to see what they would prefer you to do. Maybe a tech stop would work better.
As in so many things with aviation the answer is not black and white that is why we are paid to exercise judgement on the day.