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 $100.86   8.235%   $109.1658   $109.17   $109.15   $109.20  
  Purchase #2 $66.99   3.570%   $69.3815   $69.38   $69.40   $69.40  
  Purchase #3 $109.27   4.438%   $114.1194   $114.12   $114.10   $114.10  
  Purchase #4 $12.51   8.336%   $13.5528   $13.55   $13.55   $13.60  
  Purchase #5 $171.72   10.589%   $189.9034   $189.90   $189.90   $189.90  
  Purchase #6 $113.70   11.235%   $126.4742   $126.47   $126.45   $126.50  
  Purchase #7 $21.58   10.525%   $23.8513   $23.85   $23.85   $23.90  
  7 purchase totals = $596.63       $646.4484   $646.44   $646.40   $646.60