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 $236.51   8.361%   $256.2846   $256.28   $256.30   $256.30  
  Purchase #2 $23.75   4.479%   $24.8138   $24.81   $24.80   $24.80  
  Purchase #3 $101.53   12.597%   $114.3197   $114.32   $114.30   $114.30  
  Purchase #4 $34.33   9.792%   $37.6916   $37.69   $37.70   $37.70  
  Purchase #5 $124.54   7.474%   $133.8481   $133.85   $133.85   $133.80  
  Purchase #6 $24.87   12.536%   $27.9877   $27.99   $28.00   $28.00  
  Purchase #7 $229.53   4.548%   $239.969   $239.97   $239.95   $240.00  
  7 purchase totals = $775.06       $834.9145   $834.91   $834.90   $834.90