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 $18.66   3.612%   $19.334   $19.33   $19.35   $19.30  
  Purchase #2 $18.73   3.704%   $19.4238   $19.42   $19.40   $19.40  
  Purchase #3 $79.61   9.528%   $87.1952   $87.20   $87.20   $87.20  
  Purchase #4 $170.57   13.818%   $194.1394   $194.14   $194.15   $194.10  
  Purchase #5 $27.58   7.834%   $29.7406   $29.74   $29.75   $29.70  
  Purchase #6 $232.74   7.577%   $250.3747   $250.37   $250.35   $250.40  
  Purchase #7 $35.89   11.312%   $39.9499   $39.95   $39.95   $39.90  
  7 purchase totals = $583.78       $640.1576   $640.15   $640.15   $640.00