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 $240.92   3.488%   $249.3233   $249.32   $249.30   $249.30  
  Purchase #2 $157.75   4.314%   $164.5553   $164.56   $164.55   $164.60  
  Purchase #3 $32.92   4.187%   $34.2984   $34.30   $34.30   $34.30  
  Purchase #4 $150.90   8.220%   $163.304   $163.30   $163.30   $163.30  
  Purchase #5 $145.63   13.534%   $165.3396   $165.34   $165.35   $165.30  
  Purchase #6 $89.26   3.719%   $92.5796   $92.58   $92.60   $92.60  
  Purchase #7 $157.47   9.554%   $172.5147   $172.51   $172.50   $172.50  
  7 purchase totals = $974.85       $1,041.9149   $1,041.91   $1,041.90   $1,041.90