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 $19.99   9.267%   $21.8425   $21.84   $21.85   $21.80  
  Purchase #2 $95.62   6.440%   $101.7779   $101.78   $101.80   $101.80  
  Purchase #3 $18.47   13.742%   $21.0081   $21.01   $21.00   $21.00  
  Purchase #4 $28.93   10.188%   $31.8774   $31.88   $31.90   $31.90  
  Purchase #5 $36.81   9.565%   $40.3309   $40.33   $40.35   $40.30  
  Purchase #6 $245.38   9.296%   $268.1905   $268.19   $268.20   $268.20  
  Purchase #7 $48.38   10.242%   $53.3351   $53.34   $53.35   $53.30  
  7 purchase totals = $493.58       $538.3624   $538.37   $538.45   $538.30