Full everytime. I CBA with £10, £20 or whatever.
Plus it helps me monitor the fuel consumption and has done for years. I know with my current car I should get a minimum of 600 miles on a full tank and just over 700 miles if driving conditions have been very favourable. Same for Mrs B's car, but she is more heavy footed. If the consumption falls it makes me think is it my driving, the conditions I have been driving in, or an engine fault.
But then I play chicken in my car when the fuel warning light, which drives (sic) Mrs B up the wall.