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 $6.37   6.781%   $6.8019   $6.80   $6.80   $6.80  
  Purchase #2 $113.71   8.375%   $123.2332   $123.23   $123.25   $123.20  
  Purchase #3 $136.60   6.810%   $145.9025   $145.90   $145.90   $145.90  
  Purchase #4 $133.47   8.616%   $144.9698   $144.97   $144.95   $145.00  
  Purchase #5 $84.95   8.664%   $92.3101   $92.31   $92.30   $92.30  
  Purchase #6 $22.98   13.675%   $26.1225   $26.12   $26.10   $26.10  
  Purchase #7 $77.48   4.785%   $81.1874   $81.19   $81.20   $81.20  
  7 purchase totals = $575.56       $620.5274   $620.52   $620.50   $620.50