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 $28.56   6.798%   $30.5015   $30.50   $30.50   $30.50  
  Purchase #2 $103.28   11.533%   $115.1913   $115.19   $115.20   $115.20  
  Purchase #3 $29.25   11.430%   $32.5933   $32.59   $32.60   $32.60  
  Purchase #4 $82.45   10.102%   $90.7791   $90.78   $90.80   $90.80  
  Purchase #5 $21.89   3.708%   $22.7017   $22.70   $22.70   $22.70  
  Purchase #6 $27.47   5.337%   $28.9361   $28.94   $28.95   $28.90  
  Purchase #7 $160.46   6.413%   $170.7503   $170.75   $170.75   $170.80  
  7 purchase totals = $453.36       $491.4533   $491.45   $491.50   $491.50