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 $26.81   4.863%   $28.1138   $28.11   $28.10   $28.10  
  Purchase #2 $25.54   11.617%   $28.507   $28.51   $28.50   $28.50  
  Purchase #3 $23.55   9.507%   $25.7889   $25.79   $25.80   $25.80  
  Purchase #4 $142.94   9.706%   $156.8138   $156.81   $156.80   $156.80  
  Purchase #5 $116.75   8.490%   $126.6621   $126.66   $126.65   $126.70  
  Purchase #6 $36.57   13.715%   $41.5856   $41.59   $41.60   $41.60  
  Purchase #7 $16.97   13.380%   $19.2406   $19.24   $19.25   $19.20  
  7 purchase totals = $389.13       $426.7118   $426.71   $426.70   $426.70