First of all, you should round off when you know some part of your calculation is only correct to a certain number of significant figures, e,g if you approximate π by 3.14 somewhere in your computation, there is no justification for showing an answer to more than 3 significant figures.
Also sometimes a certain number of significant figures suffices for the purpose intended. For example in carpentry, measurements can rarely be made to less than 1/16", i.e. .0625 and quite frequently even that level of precision is needless.
Obviously, when you are working with dollars and cents, you will need to round off...and you have to do it carefully. For example, if you are computing the monthly payment for a 30 year mortgage, rounding off has to be done at the end of the computation, not in the middle of it.
I hope this helps.
