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 $89.66   11.790%   $100.2309   $100.23   $100.25   $100.20  
  Purchase #2 $101.96   12.695%   $114.9038   $114.90   $114.90   $114.90  
  Purchase #3 $100.72   5.641%   $106.4016   $106.40   $106.40   $106.40  
  Purchase #4 $29.83   8.193%   $32.274   $32.27   $32.25   $32.30  
  Purchase #5 $197.60   4.732%   $206.9504   $206.95   $206.95   $207.00  
  Purchase #6 $244.88   5.860%   $259.23   $259.23   $259.25   $259.20  
  Purchase #7 $175.49   6.161%   $186.3019   $186.30   $186.30   $186.30  
  7 purchase totals = $940.14       $1,006.2926   $1,006.28   $1,006.30   $1,006.30