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 $28.31   5.580%   $29.8897   $29.89   $29.90   $29.90  
  Purchase #2 $109.73   13.649%   $124.707   $124.71   $124.70   $124.70  
  Purchase #3 $13.38   12.747%   $15.0855   $15.09   $15.10   $15.10  
  Purchase #4 $97.71   7.523%   $105.0607   $105.06   $105.05   $105.10  
  Purchase #5 $155.45   12.211%   $174.432   $174.43   $174.45   $174.40  
  Purchase #6 $63.75   12.595%   $71.7793   $71.78   $71.80   $71.80  
  Purchase #7 $16.57   3.162%   $17.0939   $17.09   $17.10   $17.10  
  7 purchase totals = $484.90       $538.0481   $538.05   $538.10   $538.10