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 $225.65   12.147%   $253.0597   $253.06   $253.05   $253.10  
  Purchase #2 $109.58   8.212%   $118.5787   $118.58   $118.60   $118.60  
  Purchase #3 $107.48   10.783%   $119.0696   $119.07   $119.05   $119.10  
  Purchase #4 $73.97   6.634%   $78.8772   $78.88   $78.90   $78.90  
  Purchase #5 $195.45   9.324%   $213.6738   $213.67   $213.65   $213.70  
  Purchase #6 $163.88   11.849%   $183.2981   $183.30   $183.30   $183.30  
  Purchase #7 $72.25   12.523%   $81.2979   $81.30   $81.30   $81.30  
  7 purchase totals = $948.26       $1,047.8550   $1,047.86   $1,047.85   $1,048.00