You can't change values with formatting, and XL's built-in format
rounding (up and down) applies only to digits (i.e., 1s, 10s, 1/10ths).
You could use a formula:
=FLOOR(xxx,0.05)
where xxx is the current value or formula in the cell, or a reference to
another cell, e.g.:
B1: =FLOOR(A1, 0.05)