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 $18.70   6.790%   $19.9697   $19.97   $19.95   $20.00  
  Purchase #2 $98.65   8.593%   $107.127   $107.13   $107.15   $107.10  
  Purchase #3 $218.40   8.187%   $236.2804   $236.28   $236.30   $236.30  
  Purchase #4 $204.58   4.698%   $214.1912   $214.19   $214.20   $214.20  
  Purchase #5 $128.31   4.318%   $133.8504   $133.85   $133.85   $133.90  
  Purchase #6 $30.44   6.240%   $32.3395   $32.34   $32.35   $32.30  
  Purchase #7 $177.61   10.337%   $195.9695   $195.97   $195.95   $196.00  
  7 purchase totals = $876.69       $939.7277   $939.73   $939.75   $939.80