No there is no way to just have the entire sheet round all of the numbers to
the nearest 5. That being said you can just use cell references within the
formula so the amount of typeing should be minimal.
=mround(A1, 5)
where the cell you want rounded is in cell A1
Note that the above formula only works if you have the analysis toolpack
installed. If you try to use this or send it to someone who desn not have the
toolpack installed you will get errors. If that is a consideration then this
formula will work better
=round(A1/5, 0) * 5