I have a variable voltage switching power supply that outputs 11-18VDC. My charger takes 12-18VDC. As long As I keep the power supply output within the charger input range, does it matter what value I choose? Sorry for the noob question. This is my first power supply.

Good power supplies run about 13.8 volts on average.

swerv that will all depend on charger and batterys cells.. some chargers have a problem charging 4 cell on 12v. while others do not

Id run the PS V slightly higher than finished battery volts when you can

I charge 4 4s 3300mah packs simultaneously on my hitec 4 port with a ibm server power supply with absolutely no issues. power supply is 45-60w max. 3 years going strong. these are our boat packs running 4s 2p.

In general if a higher voltage is available that is preferable, generally more efficient and cooler operating. And depending on the charger and packs may allow for more power - higher amp charge rates.

One consideration, while not always necessary it's best to avoid a supply voltage in the same range as the battery pack. For example a 12V supply charging a 3S lipo causes the charger to switch between buck and boost modes as it charges. Some chargers, even some otherwise well regarded ones, are less than perfectly stable while making the transition. Generally it's not a huge issue, but not ideal, avoiding it if possible is reasonable.

Give us some more details on your equipment and packs and we can be more specific about your situation.

this sounds like very good information that i hadn't thought about. fortunately i'm only using 2S Lipo at the moment so my input voltage is considerably higher than my battery needs. Trying to understand efficiency here... Will a higher power supply output voltage decrease amperage used assuming the load stays the same?

hitec 30a power supply
http://www3.towerhobbies.com/cgi-bin/wti0001p?&I=LXDAKK

hitec X2 ultima dual port charger
http://www3.towerhobbies.com/cgi-bin...?&I=LXAYVD&P=M

charging SMC 6500maH 60c 2S Lipos

That is correct, higher output voltage means you don't have to push as many amps to get the same amount of current to pass in a given time. As previously stated, it will also be more efficient and produce less heat than a lower voltage input.