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 $19.42   7.441%   $20.865   $20.87   $20.85   $20.90  
  Purchase #2 $164.69   13.672%   $187.2064   $187.21   $187.20   $187.20  
  Purchase #3 $147.44   12.717%   $166.1899   $166.19   $166.20   $166.20  
  Purchase #4 $214.59   10.832%   $237.8344   $237.83   $237.85   $237.80  
  Purchase #5 $42.83   12.664%   $48.254   $48.25   $48.25   $48.30  
  Purchase #6 $77.26   5.258%   $81.3223   $81.32   $81.30   $81.30  
  Purchase #7 $77.72   4.313%   $81.0721   $81.07   $81.05   $81.10  
  7 purchase totals = $743.95       $822.7441   $822.74   $822.70   $822.80