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 $37.29   8.411%   $40.4265   $40.43   $40.45   $40.40  
  Purchase #2 $89.25   7.646%   $96.0741   $96.07   $96.05   $96.10  
  Purchase #3 $190.49   10.428%   $210.3543   $210.35   $210.35   $210.40  
  Purchase #4 $98.95   3.497%   $102.4103   $102.41   $102.40   $102.40  
  Purchase #5 $125.99   13.746%   $143.3086   $143.31   $143.30   $143.30  
  Purchase #6 $134.80   4.476%   $140.8336   $140.83   $140.85   $140.80  
  Purchase #7 $93.71   5.525%   $98.8875   $98.89   $98.90   $98.90  
  7 purchase totals = $770.48       $832.2949   $832.29   $832.30   $832.30