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 $15.62   5.358%   $16.4569   $16.46   $16.45   $16.50  
  Purchase #2 $62.34   8.126%   $67.4057   $67.41   $67.40   $67.40  
  Purchase #3 $46.34   9.805%   $50.8836   $50.88   $50.90   $50.90  
  Purchase #4 $158.48   9.380%   $173.3454   $173.35   $173.35   $173.30  
  Purchase #5 $138.87   12.131%   $155.7163   $155.72   $155.70   $155.70  
  Purchase #6 $31.89   4.790%   $33.4175   $33.42   $33.40   $33.40  
  Purchase #7 $137.70   3.873%   $143.0331   $143.03   $143.05   $143.00  
  7 purchase totals = $591.24       $640.2585   $640.27   $640.25   $640.20