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 $20.31   7.786%   $21.8913   $21.89   $21.90   $21.90  
  Purchase #2 $101.67   11.630%   $113.4942   $113.49   $113.50   $113.50  
  Purchase #3 $170.65   5.410%   $179.8822   $179.88   $179.90   $179.90  
  Purchase #4 $175.25   12.491%   $197.1405   $197.14   $197.15   $197.10  
  Purchase #5 $42.70   12.261%   $47.9354   $47.94   $47.95   $47.90  
  Purchase #6 $142.44   10.107%   $156.8364   $156.84   $156.85   $156.80  
  Purchase #7 $60.31   4.426%   $62.9793   $62.98   $63.00   $63.00  
  7 purchase totals = $713.33       $780.1593   $780.16   $780.25   $780.10