We had a discussion about sig figs a few pages back. Basically decided that you can only be penalised for rounding errors once per paper, where it tells you sig figs are important
basically it's just good practice to use exact answers wherever possible in the middle of your working - i.e, if you have to multiply by a number you've worked out, use the whole number, not a rounded one, or you'll get errors. use the store/retrieve function on your calculator if the answers are just long strings of decimals.