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 $19.67   4.635%   $20.5817   $20.58   $20.60   $20.60  
  Purchase #2 $82.70   9.829%   $90.8286   $90.83   $90.85   $90.80  
  Purchase #3 $121.77   8.541%   $132.1704   $132.17   $132.15   $132.20  
  Purchase #4 $199.87   5.106%   $210.0754   $210.08   $210.10   $210.10  
  Purchase #5 $178.97   3.377%   $185.0138   $185.01   $185.00   $185.00  
  Purchase #6 $16.74   13.237%   $18.9559   $18.96   $18.95   $19.00  
  Purchase #7 $144.80   13.763%   $164.7288   $164.73   $164.75   $164.70  
  7 purchase totals = $764.52       $822.3546   $822.36   $822.40   $822.40