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 $18.32   9.124%   $19.9915   $19.99   $20.00   $20.00  
  Purchase #2 $228.43   13.200%   $258.5828   $258.58   $258.60   $258.60  
  Purchase #3 $16.67   8.858%   $18.1466   $18.15   $18.15   $18.10  
  Purchase #4 $210.89   11.179%   $234.4654   $234.47   $234.45   $234.50  
  Purchase #5 $56.27   7.302%   $60.3788   $60.38   $60.40   $60.40  
  Purchase #6 $38.99   13.432%   $44.2271   $44.23   $44.25   $44.20  
  Purchase #7 $19.57   7.599%   $21.0571   $21.06   $21.05   $21.10  
  7 purchase totals = $589.14       $656.8493   $656.86   $656.90   $656.90