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 $27.47   10.610%   $30.3846   $30.38   $30.40   $30.40  
  Purchase #2 $16.62   4.299%   $17.3345   $17.33   $17.35   $17.30  
  Purchase #3 $158.99   6.777%   $169.7648   $169.76   $169.75   $169.80  
  Purchase #4 $53.84   13.106%   $60.8963   $60.90   $60.90   $60.90  
  Purchase #5 $247.43   8.342%   $268.0706   $268.07   $268.05   $268.10  
  Purchase #6 $139.56   12.640%   $157.2004   $157.20   $157.20   $157.20  
  Purchase #7 $79.34   12.654%   $89.3797   $89.38   $89.40   $89.40  
  7 purchase totals = $723.25       $793.0309   $793.02   $793.05   $793.10