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 $34.61   11.620%   $38.6317   $38.63   $38.65   $38.60  
  Purchase #2 $148.73   10.789%   $164.7765   $164.78   $164.80   $164.80  
  Purchase #3 $239.79   10.657%   $265.3444   $265.34   $265.35   $265.30  
  Purchase #4 $70.61   8.166%   $76.376   $76.38   $76.40   $76.40  
  Purchase #5 $84.57   4.653%   $88.505   $88.51   $88.50   $88.50  
  Purchase #6 $76.93   11.261%   $85.5931   $85.59   $85.60   $85.60  
  Purchase #7 $114.58   3.368%   $118.4391   $118.44   $118.45   $118.40  
  7 purchase totals = $769.82       $837.6658   $837.67   $837.75   $837.60