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 $249.91   13.732%   $284.2276   $284.23   $284.25   $284.20  
  Purchase #2 $51.38   8.530%   $55.7627   $55.76   $55.75   $55.80  
  Purchase #3 $233.69   6.767%   $249.5038   $249.50   $249.50   $249.50  
  Purchase #4 $76.80   12.316%   $86.2587   $86.26   $86.25   $86.30  
  Purchase #5 $82.72   6.738%   $88.2937   $88.29   $88.30   $88.30  
  Purchase #6 $186.36   13.335%   $211.2111   $211.21   $211.20   $211.20  
  Purchase #7 $247.38   11.235%   $275.1731   $275.17   $275.15   $275.20  
  7 purchase totals = $1,128.24       $1,250.4307   $1,250.42   $1,250.40   $1,250.50