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 $40.28   4.108%   $41.9347   $41.93   $41.95   $41.90  
  Purchase #2 $90.57   10.167%   $99.7783   $99.78   $99.80   $99.80  
  Purchase #3 $236.91   11.637%   $264.4792   $264.48   $264.50   $264.50  
  Purchase #4 $129.80   7.670%   $139.7557   $139.76   $139.75   $139.80  
  Purchase #5 $10.37   10.350%   $11.4433   $11.44   $11.45   $11.40  
  Purchase #6 $20.48   3.595%   $21.2163   $21.22   $21.20   $21.20  
  Purchase #7 $9.75   13.787%   $11.0942   $11.09   $11.10   $11.10  
  7 purchase totals = $538.16       $589.7017   $589.70   $589.75   $589.70