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 $248.91   6.677%   $265.5297   $265.53   $265.55   $265.50  
  Purchase #2 $19.96   8.153%   $21.5873   $21.59   $21.60   $21.60  
  Purchase #3 $14.45   10.463%   $15.9619   $15.96   $15.95   $16.00  
  Purchase #4 $137.41   6.351%   $146.1369   $146.14   $146.15   $146.10  
  Purchase #5 $230.72   8.674%   $250.7327   $250.73   $250.75   $250.70  
  Purchase #6 $143.89   12.869%   $162.4072   $162.41   $162.40   $162.40  
  Purchase #7 $55.59   8.266%   $60.1851   $60.19   $60.20   $60.20  
  7 purchase totals = $850.93       $922.5408   $922.55   $922.60   $922.50