Standard Practice for including custom library in examples folder
You can use either. There is no "standard".

When using a raw C compiler there is a difference between the two as regards the order in which directories are searched for files to include, but with the Arduino that is completely irrelevant, so either can be used.

Personally I use &lt;...&gt; for libraries and "..." for header files that form part of the sketch.  It makes for easier identification of which is which, and conforms to the C standard of &lt;...&gt; for system-installed headers, and "..." for local headers.