Line tool option in tool option bar create shape layers so you can position your arrows and strike the mark
How about if I draw two shape lines to make a 90 degree angle, but I only want one of the line to have the arrow?
Is there a way to draw all my lines, then select the ones that I want to add arrowheads? it would be highly unintuitive if one has to check the "add arrow" to draw a line with arrow, then uncheck to draw a line without the arrow and so on.
Another option wort remembering are the custom shapes.
I can't remember if the arrows are in the default collection, and I usually have mine set to All
Yep. There you go. The Arrows have their own set, but like I say, I can't really think of any down side of having them all loaded, unless your system is very short of RAM.
Yet another option is to draw an arrow and save it as a custom shape from the Edit menu. It will appear in the Shape dropdown as shown in Trevor's screen shot. You can use it to draw a Shape, Path, or Pixels, depending on what you select in the Options bar.