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 $250.58   11.479%   $279.3441   $279.34   $279.35   $279.30  
  Purchase #2 $58.77   7.691%   $63.29   $63.29   $63.30   $63.30  
  Purchase #3 $217.67   4.504%   $227.4739   $227.47   $227.45   $227.50  
  Purchase #4 $186.74   4.403%   $194.9622   $194.96   $194.95   $195.00  
  Purchase #5 $146.53   13.605%   $166.4654   $166.47   $166.45   $166.50  
  Purchase #6 $26.80   13.827%   $30.5056   $30.51   $30.50   $30.50  
  Purchase #7 $90.44   13.802%   $102.9225   $102.92   $102.90   $102.90  
  7 purchase totals = $977.53       $1,064.9637   $1,064.96   $1,064.90   $1,065.00