According to :

I wouldn't worry about one for an air conditioner. Just make sure it
has a proper ground. You can buy a simple plug in outlet tester that
will tell you if the outlet is wired properly, has a ground, etc.

It's probably true that A/Cs are a little more likely to have
certain kinds of ground faults (eg: as a result of condensation)
than many other devices. Given their size and that they're mostly
metal, the likelyhood of experiencing a shock if a fault occurs
will be higher than with most other things.

Similarly with refrigerators - it's not at all unusual for
old fridges to have ground faults. However, they're less likely
to put GFCI on fridges, because the consequences of a false trip
are rather worse than with an A/C.

If the OP does plan on GFCI'ing the A/Cs, he has several options:

- GFCI breaker
- GFCI outlet
- GFCI "cord".

The latter two are really only applicable to plug-in 120V A/Cs.
A 120V GFCI outlet is $10. A GFCI "cord" is typically about
$25, and doesn't require changing any permanent wiring (no
"installation"). It's just a short extension cord with a GFCI
in it.

If you do go with the "GFCI cord" approach, make sure that the
wire itself is at least 14ga or 12ga (depending on whether
the AC is 12A or 16A).

A GFCI breaker could set you back anywhere from about $20 to
$200 (_not_ including installation), depending on the panel,
the amperage and whether it's 120V or 240V. 240V GFCI breakers
can sometimes be hideously expensive.
--
Chris Lewis,

Age and Treachery will Triumph over Youth and Skill
It's not just anyone who gets a Starship Cruiser class named after them.