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 $241.58   3.280%   $249.5038   $249.50   $249.50   $249.50  
  Purchase #2 $169.51   3.407%   $175.2852   $175.29   $175.30   $175.30  
  Purchase #3 $123.31   4.133%   $128.4064   $128.41   $128.40   $128.40  
  Purchase #4 $125.80   11.829%   $140.6809   $140.68   $140.70   $140.70  
  Purchase #5 $177.45   6.768%   $189.4598   $189.46   $189.45   $189.50  
  Purchase #6 $110.39   5.179%   $116.1071   $116.11   $116.10   $116.10  
  Purchase #7 $38.29   6.653%   $40.8374   $40.84   $40.85   $40.80  
  7 purchase totals = $986.33       $1,040.2806   $1,040.29   $1,040.30   $1,040.30