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 $18.41   12.645%   $20.7379   $20.74   $20.75   $20.70  
  Purchase #2 $203.69   10.825%   $225.7394   $225.74   $225.75   $225.70  
  Purchase #3 $18.40   10.378%   $20.3096   $20.31   $20.30   $20.30  
  Purchase #4 $100.87   12.193%   $113.1691   $113.17   $113.15   $113.20  
  Purchase #5 $42.95   5.316%   $45.2332   $45.23   $45.25   $45.20  
  Purchase #6 $62.79   9.360%   $68.6671   $68.67   $68.65   $68.70  
  Purchase #7 $82.38   5.679%   $87.0584   $87.06   $87.05   $87.10  
  7 purchase totals = $529.49       $580.9147   $580.92   $580.90   $580.90