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 $28.33   9.311%   $30.9678   $30.97   $30.95   $31.00  
  Purchase #2 $226.29   5.611%   $238.9871   $238.99   $239.00   $239.00  
  Purchase #3 $117.88   7.625%   $126.8684   $126.87   $126.85   $126.90  
  Purchase #4 $131.27   6.622%   $139.9627   $139.96   $139.95   $140.00  
  Purchase #5 $209.87   9.375%   $229.5453   $229.55   $229.55   $229.50  
  Purchase #6 $36.48   13.378%   $41.3603   $41.36   $41.35   $41.40  
  Purchase #7 $55.58   13.303%   $62.9738   $62.97   $62.95   $63.00  
  7 purchase totals = $805.70       $870.6654   $870.67   $870.60   $870.80