Yes I always use the birth name no matter how many times people marry. I've noticed in some people's trees they have put married names but may be they've added something and not realised the surname has changed due to the record added and they have n't realised. Although have to be wary as they may be cousins who have married or surname the same by chance. If you go down the DNA route as well, it may cause you difficulties if you have n't got correct surnames on!