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 $18.96   10.716%   $20.9918   $20.99   $21.00   $21.00  
  Purchase #2 $105.79   8.742%   $115.0382   $115.04   $115.05   $115.00  
  Purchase #3 $112.41   13.772%   $127.8911   $127.89   $127.90   $127.90  
  Purchase #4 $77.64   6.704%   $82.845   $82.85   $82.85   $82.80  
  Purchase #5 $132.89   4.321%   $138.6322   $138.63   $138.65   $138.60  
  Purchase #6 $22.93   9.745%   $25.1645   $25.16   $25.15   $25.20  
  Purchase #7 $226.62   3.115%   $233.6792   $233.68   $233.70   $233.70  
  7 purchase totals = $697.24       $744.2420   $744.24   $744.30   $744.20