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 $246.46   12.306%   $276.7894   $276.79   $276.80   $276.80  
  Purchase #2 $217.68   13.653%   $247.3999   $247.40   $247.40   $247.40  
  Purchase #3 $15.42   13.254%   $17.4638   $17.46   $17.45   $17.50  
  Purchase #4 $166.91   7.660%   $179.6953   $179.70   $179.70   $179.70  
  Purchase #5 $164.77   8.584%   $178.9139   $178.91   $178.90   $178.90  
  Purchase #6 $244.38   6.293%   $259.7588   $259.76   $259.75   $259.80  
  Purchase #7 $163.29   8.202%   $176.683   $176.68   $176.70   $176.70  
  7 purchase totals = $1,218.91       $1,336.7041   $1,336.70   $1,336.70   $1,336.80