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 $107.50   12.421%   $120.8526   $120.85   $120.85   $120.90  
  Purchase #2 $107.32   3.420%   $110.9903   $110.99   $111.00   $111.00  
  Purchase #3 $135.63   4.528%   $141.7713   $141.77   $141.75   $141.80  
  Purchase #4 $82.56   10.440%   $91.1793   $91.18   $91.20   $91.20  
  Purchase #5 $28.64   7.408%   $30.7617   $30.76   $30.75   $30.80  
  Purchase #6 $40.38   9.304%   $44.137   $44.14   $44.15   $44.10  
  Purchase #7 $14.38   12.590%   $16.1904   $16.19   $16.20   $16.20  
  7 purchase totals = $516.41       $555.8826   $555.88   $555.90   $556.00