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 $90.43   8.781%   $98.3707   $98.37   $98.35   $98.40  
  Purchase #2 $59.74   3.725%   $61.9653   $61.97   $61.95   $62.00  
  Purchase #3 $125.37   13.861%   $142.7475   $142.75   $142.75   $142.70  
  Purchase #4 $219.71   9.855%   $241.3624   $241.36   $241.35   $241.40  
  Purchase #5 $180.64   4.490%   $188.7507   $188.75   $188.75   $188.80  
  Purchase #6 $117.94   5.413%   $124.3241   $124.32   $124.30   $124.30  
  Purchase #7 $60.35   6.117%   $64.0416   $64.04   $64.05   $64.00  
  7 purchase totals = $854.18       $921.5623   $921.56   $921.50   $921.60