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 $38.38   5.213%   $40.3807   $40.38   $40.40   $40.40  
  Purchase #2 $104.87   12.288%   $117.7564   $117.76   $117.75   $117.80  
  Purchase #3 $30.44   4.237%   $31.7297   $31.73   $31.75   $31.70  
  Purchase #4 $25.30   6.712%   $26.9981   $27.00   $27.00   $27.00  
  Purchase #5 $227.67   11.521%   $253.8999   $253.90   $253.90   $253.90  
  Purchase #6 $157.87   8.828%   $171.8068   $171.81   $171.80   $171.80  
  Purchase #7 $60.89   9.295%   $66.5497   $66.55   $66.55   $66.50  
  7 purchase totals = $645.42       $709.1213   $709.13   $709.15   $709.10