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 $235.58   6.325%   $250.4804   $250.48   $250.50   $250.50  
  Purchase #2 $203.58   10.677%   $225.3162   $225.32   $225.30   $225.30  
  Purchase #3 $60.71   13.458%   $68.8804   $68.88   $68.90   $68.90  
  Purchase #4 $28.92   11.659%   $32.2918   $32.29   $32.30   $32.30  
  Purchase #5 $91.29   6.820%   $97.516   $97.52   $97.50   $97.50  
  Purchase #6 $58.38   13.618%   $66.3302   $66.33   $66.35   $66.30  
  Purchase #7 $61.69   6.523%   $65.714   $65.71   $65.70   $65.70  
  7 purchase totals = $740.15       $806.5290   $806.53   $806.55   $806.50