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 $19.28   11.113%   $21.4226   $21.42   $21.40   $21.40  
  Purchase #2 $14.31   12.604%   $16.1136   $16.11   $16.10   $16.10  
  Purchase #3 $131.77   7.367%   $141.4775   $141.48   $141.50   $141.50  
  Purchase #4 $65.37   7.393%   $70.2028   $70.20   $70.20   $70.20  
  Purchase #5 $72.94   12.795%   $82.2727   $82.27   $82.25   $82.30  
  Purchase #6 $21.41   12.324%   $24.0486   $24.05   $24.05   $24.00  
  Purchase #7 $161.30   13.187%   $182.5706   $182.57   $182.55   $182.60  
  7 purchase totals = $486.38       $538.1084   $538.10   $538.05   $538.10