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 $16.73   10.623%   $18.5072   $18.51   $18.50   $18.50  
  Purchase #2 $185.61   9.703%   $203.6197   $203.62   $203.60   $203.60  
  Purchase #3 $58.64   12.488%   $65.963   $65.96   $65.95   $66.00  
  Purchase #4 $78.89   7.119%   $84.5062   $84.51   $84.50   $84.50  
  Purchase #5 $61.50   9.551%   $67.3739   $67.37   $67.35   $67.40  
  Purchase #6 $202.59   4.778%   $212.2698   $212.27   $212.25   $212.30  
  Purchase #7 $13.58   8.549%   $14.741   $14.74   $14.75   $14.70  
  7 purchase totals = $617.54       $666.9808   $666.98   $666.90   $667.00