This expands when defined, not when used. Consider escaping.

Problematic code:

alias whereami="echo $PWD"

Correct code:

alias whereami='echo $PWD'

Rationale:

With double quotes, this particular alias will be defined as echo /home/me, so it will always print the same path. This is rarely intended.

By using single quotes or escaping any expansions, we define the alias as echo $PWD, which will be expanded when we use the alias. This is the far more common use case.

Note that even if you expect that the variable will never change, it may still be better to quote it. This prevents a second round of evaluation later:

default="Can't handle failure, aborting"
trap "echo $default; exit 1" err
false 

The trap now has a syntax error, because instead of running echo $default, it runs echo Can't handle .. which has an unmatched single quote. Avoid early expansion unless you're equally comfortable putting eval in there.

Exceptions

If you don't mind that your alias definition is expanded at define time (and its result expanded again at evaluation time), you can ignore this warning.