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 $84.32   10.671%   $93.3178   $93.32   $93.30   $93.30  
  Purchase #2 $108.89   5.493%   $114.8713   $114.87   $114.85   $114.90  
  Purchase #3 $28.48   3.507%   $29.4788   $29.48   $29.50   $29.50  
  Purchase #4 $182.71   3.569%   $189.2309   $189.23   $189.25   $189.20  
  Purchase #5 $62.94   4.148%   $65.5508   $65.55   $65.55   $65.60  
  Purchase #6 $8.42   8.624%   $9.1461   $9.15   $9.15   $9.10  
  Purchase #7 $62.40   12.674%   $70.3086   $70.31   $70.30   $70.30  
  7 purchase totals = $538.16       $571.9043   $571.91   $571.90   $571.90