Impedance - 5channel amps

Is it safe to run an amp which has 5 channels in the following configuration?

4 x channels @ 4ohms  +  1 x channel @ 2ohms    or

2 x channels @ 4ohms  +  1 x channel @ 2ohms

Ive only seen amps rated across the channels at 4ohms or 2ohms but not both - with the exception of JL Audio which lists only one continuous power from 1.5 - 4 ohms.

Here are some examples:

Page 8 of the Fosgate PDF manual :   http://www.rockfordfosgate.com/ezcompare/ExternalFiles/Rockford/2001/2001_PunchMultiCh_MAN.pdf

   http://www.jlaudio.com/amps/5005.html

What are the odds of having the same question on the same day??  Guess I shoulda looked first before posting....

I have a general question about 5 channel amps.  Let's say a 5 channel amp has the following power specs:

50W x 4 + 150W x 1 @ 4 ohms

75W x 4 + 250W x 1 @ 2 ohms

Does this generally mean I can run 50W to each of my 4 ohm speakers, then wire my subs to create a 2 ohm load and push them at 250W?  Can I do that; can I split-up the ratings the amp has based on how I want to use each of the channels?  I know not all amps are the same, and some older models had a switch for 4 ohm and 2 ohm mode.  No amp specs I have seen give 4 channels at 4 ohms, and then the sub- channel at 2 ohm.  Please advise...