Let's do some math ..  Rounding examples of 7 random hypothetical purchases:
  Case Tag  
Price  
Sales  
Tax %  
Raw Total  
Incl. Sales Tax  
Rounded  
to Penny  
Rounded  
to Nickel  
Rounded  
to Dime  
  Purchase #1 $225.89   12.227%   $253.5096   $253.51   $253.50   $253.50  
  Purchase #2 $142.95   5.665%   $151.0481   $151.05   $151.05   $151.00  
  Purchase #3 $57.34   7.160%   $61.4455   $61.45   $61.45   $61.40  
  Purchase #4 $38.30   8.325%   $41.4885   $41.49   $41.50   $41.50  
  Purchase #5 $19.66   13.323%   $22.2793   $22.28   $22.30   $22.30  
  Purchase #6 $68.50   11.323%   $76.2563   $76.26   $76.25   $76.30  
  Purchase #7 $186.27   10.220%   $205.3068   $205.31   $205.30   $205.30  
  7 purchase totals = $738.91       $811.3341   $811.35   $811.35   $811.30