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 $28.70   8.825%   $31.2328   $31.23   $31.25   $31.20  
  Purchase #2 $215.31   10.649%   $238.2384   $238.24   $238.25   $238.20  
  Purchase #3 $96.27   5.660%   $101.7189   $101.72   $101.70   $101.70  
  Purchase #4 $126.47   4.510%   $132.1738   $132.17   $132.15   $132.20  
  Purchase #5 $137.65   9.640%   $150.9195   $150.92   $150.90   $150.90  
  Purchase #6 $105.75   4.691%   $110.7107   $110.71   $110.70   $110.70  
  Purchase #7 $211.76   6.676%   $225.8971   $225.90   $225.90   $225.90  
  7 purchase totals = $921.91       $990.8912   $990.89   $990.85   $990.80