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 $16.40   10.312%   $18.0912   $18.09   $18.10   $18.10  
  Purchase #2 $51.49   12.430%   $57.8902   $57.89   $57.90   $57.90  
  Purchase #3 $6.48   6.526%   $6.9029   $6.90   $6.90   $6.90  
  Purchase #4 $116.44   6.584%   $124.1064   $124.11   $124.10   $124.10  
  Purchase #5 $86.72   6.821%   $92.6352   $92.64   $92.65   $92.60  
  Purchase #6 $22.66   4.454%   $23.6693   $23.67   $23.65   $23.70  
  Purchase #7 $155.72   13.823%   $177.2452   $177.25   $177.25   $177.20  
  7 purchase totals = $455.91       $500.5404   $500.55   $500.55   $500.50