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 $20.61   4.606%   $21.5593   $21.56   $21.55   $21.60  
  Purchase #2 $197.63   9.694%   $216.7883   $216.79   $216.80   $216.80  
  Purchase #3 $45.61   7.535%   $49.0467   $49.05   $49.05   $49.00  
  Purchase #4 $87.92   12.565%   $98.9671   $98.97   $98.95   $99.00  
  Purchase #5 $43.34   12.281%   $48.6626   $48.66   $48.65   $48.70  
  Purchase #6 $83.86   6.594%   $89.3897   $89.39   $89.40   $89.40  
  Purchase #7 $35.55   3.248%   $36.7047   $36.70   $36.70   $36.70  
  7 purchase totals = $514.52       $561.1184   $561.12   $561.10   $561.20