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 $93.40   12.535%   $105.1077   $105.11   $105.10   $105.10  
  Purchase #2 $78.82   11.808%   $88.1271   $88.13   $88.15   $88.10  
  Purchase #3 $125.26   9.453%   $137.1008   $137.10   $137.10   $137.10  
  Purchase #4 $143.61   9.503%   $157.2573   $157.26   $157.25   $157.30  
  Purchase #5 $66.36   8.143%   $71.7637   $71.76   $71.75   $71.80  
  Purchase #6 $28.99   6.678%   $30.926   $30.93   $30.95   $30.90  
  Purchase #7 $47.53   3.596%   $49.2392   $49.24   $49.25   $49.20  
  7 purchase totals = $583.97       $639.5218   $639.53   $639.55   $639.50