Is using "iff" appropriate?
The abbreviation iff is generally understood in several fields outside logic, and in my experience it is well understood by physicists, but the golden rule is to define every abbreviation at its first use and/or provide a list of abbreviations. In this way, even those not familiar with an abbreviation will be able to understand the text.
However, in general, abbreviations may make a text less readable and therefore should be used judiciously. Thus, ask yourself if you will have to use the expression if and only if so frequently to warrant an abbreviation, sparing you just a bunch of characters, and think whether to limit its use to e.g. theorem statements.