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 $16.81   12.840%   $18.9684   $18.97   $18.95   $19.00  
  Purchase #2 $10.66   13.375%   $12.0858   $12.09   $12.10   $12.10  
  Purchase #3 $42.27   8.167%   $45.7222   $45.72   $45.70   $45.70  
  Purchase #4 $39.37   13.625%   $44.7342   $44.73   $44.75   $44.70  
  Purchase #5 $165.62   3.152%   $170.8403   $170.84   $170.85   $170.80  
  Purchase #6 $127.65   9.624%   $139.935   $139.94   $139.95   $139.90  
  Purchase #7 $64.40   8.110%   $69.6228   $69.62   $69.60   $69.60  
  7 purchase totals = $466.78       $501.9087   $501.91   $501.90   $501.80