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 $56.31   11.667%   $62.8797   $62.88   $62.90   $62.90  
  Purchase #2 $19.69   7.440%   $21.1549   $21.15   $21.15   $21.20  
  Purchase #3 $198.68   8.197%   $214.9658   $214.97   $214.95   $215.00  
  Purchase #4 $62.60   10.388%   $69.1029   $69.10   $69.10   $69.10  
  Purchase #5 $121.55   6.656%   $129.6404   $129.64   $129.65   $129.60  
  Purchase #6 $54.63   5.249%   $57.4975   $57.50   $57.50   $57.50  
  Purchase #7 $65.47   12.347%   $73.5536   $73.55   $73.55   $73.60  
  7 purchase totals = $578.93       $628.7948   $628.79   $628.80   $628.90