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 $249.72   6.228%   $265.2726   $265.27   $265.25   $265.30  
  Purchase #2 $137.25   4.666%   $143.6541   $143.65   $143.65   $143.70  
  Purchase #3 $228.26   9.256%   $249.3877   $249.39   $249.40   $249.40  
  Purchase #4 $124.91   11.822%   $139.6769   $139.68   $139.70   $139.70  
  Purchase #5 $194.91   12.159%   $218.6091   $218.61   $218.60   $218.60  
  Purchase #6 $165.65   7.687%   $178.3835   $178.38   $178.40   $178.40  
  Purchase #7 $53.39   7.576%   $57.4348   $57.43   $57.45   $57.40  
  7 purchase totals = $1,154.09       $1,252.4187   $1,252.41   $1,252.45   $1,252.50