"It depends" is the answer.  On the face of it, when switched off the air
conditioning system adds weight and slightly reduces the efficiency of the
cooling system (because the condenser sits in the way of the radiator).  The
weight difference is just a few pies, you won't notice an improvement.

When running, the compressor does sap power, which in turn increases fuel
consumption.  How much will depend on vehicle and environment specifics: in
high temperatures at low speeds it'll make a significant difference.  When
it's cool and you're on the motorway (freeway) it won't make a noticeable
difference.

If you want to reduce fuel consumption, putting the tyres at the maximum
load pressures will help a bit.  Regular oil changes too.  Next time you
replace tyres, go for ones that have a low rolling resistance.  Adopt a
slower cruising speed.  Heh - plenty more fuel consumption "tricks" on my
website...

Weight saving aside, the A/C isn't drawing any power from the engine
when it's turned off, so it shouldn't be an issue.  If you really want
to remove that very small bit of drag it puts on things, removing the
A/C compressor drive belt may gain you 0.01 mpg.  At most, removing the
components will save a bit of weight.

The real catch here is that removing it improperly will allow the
refrigerant to escape, which if your system is still using freon, is a
BIG no-no.  To be done properly, an A/C shop needs to drain off the
refrigerant.  At that point, you're negated any cost savings you'd get
from the miniscule improved mileage.

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