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 $239.25   12.159%   $268.3404   $268.34   $268.35   $268.30  
  Purchase #2 $119.99   4.473%   $125.3572   $125.36   $125.35   $125.40  
  Purchase #3 $183.34   9.535%   $200.8215   $200.82   $200.80   $200.80  
  Purchase #4 $98.56   13.330%   $111.698   $111.70   $111.70   $111.70  
  Purchase #5 $77.37   5.395%   $81.5441   $81.54   $81.55   $81.50  
  Purchase #6 $177.26   3.207%   $182.9447   $182.94   $182.95   $182.90  
  Purchase #7 $156.80   7.485%   $168.5365   $168.54   $168.55   $168.50  
  7 purchase totals = $1,052.57       $1,139.2424   $1,139.24   $1,139.25   $1,139.10