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 $237.54   8.591%   $257.9471   $257.95   $257.95   $257.90  
  Purchase #2 $34.39   6.339%   $36.57   $36.57   $36.55   $36.60  
  Purchase #3 $114.51   7.503%   $123.1017   $123.10   $123.10   $123.10  
  Purchase #4 $200.95   13.519%   $228.1164   $228.12   $228.10   $228.10  
  Purchase #5 $89.41   13.176%   $101.1907   $101.19   $101.20   $101.20  
  Purchase #6 $81.73   12.424%   $91.8841   $91.88   $91.90   $91.90  
  Purchase #7 $143.46   7.185%   $153.7676   $153.77   $153.75   $153.80  
  7 purchase totals = $901.99       $992.5776   $992.58   $992.55   $992.60