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 $19.54   7.605%   $21.026   $21.03   $21.05   $21.00  
  Purchase #2 $173.79   3.707%   $180.2324   $180.23   $180.25   $180.20  
  Purchase #3 $124.62   13.866%   $141.8998   $141.90   $141.90   $141.90  
  Purchase #4 $117.37   3.456%   $121.4263   $121.43   $121.45   $121.40  
  Purchase #5 $105.86   9.558%   $115.9781   $115.98   $116.00   $116.00  
  Purchase #6 $39.90   12.661%   $44.9517   $44.95   $44.95   $45.00  
  Purchase #7 $6.27   5.561%   $6.6187   $6.62   $6.60   $6.60  
  7 purchase totals = $587.35       $632.1330   $632.14   $632.20   $632.10