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 $250.98   9.826%   $275.6413   $275.64   $275.65   $275.60  
  Purchase #2 $86.41   13.644%   $98.1998   $98.20   $98.20   $98.20  
  Purchase #3 $212.33   11.636%   $237.0367   $237.04   $237.05   $237.00  
  Purchase #4 $102.30   10.640%   $113.1847   $113.18   $113.20   $113.20  
  Purchase #5 $58.38   3.824%   $60.6125   $60.61   $60.60   $60.60  
  Purchase #6 $249.70   13.163%   $282.568   $282.57   $282.55   $282.60  
  Purchase #7 $205.73   3.656%   $213.2515   $213.25   $213.25   $213.30  
  7 purchase totals = $1,165.83       $1,280.4945   $1,280.49   $1,280.50   $1,280.50