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 $58.27   12.321%   $65.4494   $65.45   $65.45   $65.40  
  Purchase #2 $69.98   13.825%   $79.6547   $79.65   $79.65   $79.70  
  Purchase #3 $108.34   12.533%   $121.9183   $121.92   $121.90   $121.90  
  Purchase #4 $64.95   8.388%   $70.398   $70.40   $70.40   $70.40  
  Purchase #5 $117.46   10.732%   $130.0658   $130.07   $130.05   $130.10  
  Purchase #6 $115.45   8.541%   $125.3106   $125.31   $125.30   $125.30  
  Purchase #7 $141.52   9.706%   $155.2559   $155.26   $155.25   $155.30  
  7 purchase totals = $675.97       $748.0527   $748.06   $748.00   $748.10