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 $238.82   11.381%   $266.0001   $266.00   $266.00   $266.00  
  Purchase #2 $8.59   5.837%   $9.0914   $9.09   $9.10   $9.10  
  Purchase #3 $65.39   12.729%   $73.7135   $73.71   $73.70   $73.70  
  Purchase #4 $64.73   6.340%   $68.8339   $68.83   $68.85   $68.80  
  Purchase #5 $186.25   4.801%   $195.1919   $195.19   $195.20   $195.20  
  Purchase #6 $55.63   11.479%   $62.0158   $62.02   $62.00   $62.00  
  Purchase #7 $235.82   12.142%   $264.4533   $264.45   $264.45   $264.50  
  7 purchase totals = $855.23       $939.2999   $939.29   $939.30   $939.30