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 $53.43   7.322%   $57.3421   $57.34   $57.35   $57.30  
  Purchase #2 $115.50   8.821%   $125.6883   $125.69   $125.70   $125.70  
  Purchase #3 $38.45   6.429%   $40.922   $40.92   $40.90   $40.90  
  Purchase #4 $211.41   4.555%   $221.0397   $221.04   $221.05   $221.00  
  Purchase #5 $155.86   11.578%   $173.9055   $173.91   $173.90   $173.90  
  Purchase #6 $143.76   10.756%   $159.2228   $159.22   $159.20   $159.20  
  Purchase #7 $58.39   6.383%   $62.117   $62.12   $62.10   $62.10  
  7 purchase totals = $776.80       $840.2374   $840.24   $840.20   $840.10