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 $17.99   7.480%   $19.3357   $19.34   $19.35   $19.30  
  Purchase #2 $89.31   8.717%   $97.0952   $97.10   $97.10   $97.10  
  Purchase #3 $124.93   3.219%   $128.9515   $128.95   $128.95   $129.00  
  Purchase #4 $77.75   12.199%   $87.2347   $87.23   $87.25   $87.20  
  Purchase #5 $76.37   11.184%   $84.9112   $84.91   $84.90   $84.90  
  Purchase #6 $24.47   13.219%   $27.7047   $27.70   $27.70   $27.70  
  Purchase #7 $39.76   11.861%   $44.4759   $44.48   $44.50   $44.50  
  7 purchase totals = $450.58       $489.7089   $489.71   $489.75   $489.70