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 $239.73   12.189%   $268.9507   $268.95   $268.95   $269.00  
  Purchase #2 $71.72   6.356%   $76.2785   $76.28   $76.30   $76.30  
  Purchase #3 $226.87   5.464%   $239.2662   $239.27   $239.25   $239.30  
  Purchase #4 $233.51   6.128%   $247.8195   $247.82   $247.80   $247.80  
  Purchase #5 $118.67   8.680%   $128.9706   $128.97   $128.95   $129.00  
  Purchase #6 $114.81   11.799%   $128.3564   $128.36   $128.35   $128.40  
  Purchase #7 $214.74   8.372%   $232.718   $232.72   $232.70   $232.70  
  7 purchase totals = $1,220.05       $1,322.3599   $1,322.37   $1,322.30   $1,322.50