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 $36.82   8.605%   $39.9884   $39.99   $40.00   $40.00  
  Purchase #2 $179.66   7.216%   $192.6243   $192.62   $192.60   $192.60  
  Purchase #3 $130.67   10.351%   $144.1957   $144.20   $144.20   $144.20  
  Purchase #4 $188.36   6.120%   $199.8876   $199.89   $199.90   $199.90  
  Purchase #5 $13.78   3.160%   $14.2154   $14.22   $14.20   $14.20  
  Purchase #6 $10.99   6.209%   $11.6724   $11.67   $11.65   $11.70  
  Purchase #7 $49.81   3.264%   $51.4358   $51.44   $51.45   $51.40  
  7 purchase totals = $610.09       $654.0196   $654.03   $654.00   $654.00