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 $30.80   8.377%   $33.3801   $33.38   $33.40   $33.40  
  Purchase #2 $33.25   11.623%   $37.1146   $37.11   $37.10   $37.10  
  Purchase #3 $237.55   10.264%   $261.9321   $261.93   $261.95   $261.90  
  Purchase #4 $191.62   9.803%   $210.4045   $210.40   $210.40   $210.40  
  Purchase #5 $10.42   13.115%   $11.7866   $11.79   $11.80   $11.80  
  Purchase #6 $33.31   5.692%   $35.206   $35.21   $35.20   $35.20  
  Purchase #7 $46.74   3.499%   $48.3754   $48.38   $48.40   $48.40  
  7 purchase totals = $583.69       $638.1993   $638.20   $638.25   $638.20