Battery switcher? Carbon batteries

[sfr]

Okay, I've never tried it, but I picked up some carbon batteries today at the music shop.  (Snagged a used Fulltone Deluxe Wah for a great price, and the rather condescending fellow behind the counter basically told me I was stupid if I used a power supply or alkalines or anything but a carbon battery.  I appreciate that he wants to inform people, but he was a little rude about it.)

Now, from reading here and other places, I understand how the internal resistance of the battery can make a difference in some effects like FF's, etc.  But how can I tell what effects it would actually make a difference in?  Is there a rule of thumb for this stuff?  Do I measure the resistance across the +9V and ground of a pedal and make a determination from there?

Anyway, I'm going to experiment with different batteries in my FF and new wah and see what I'd like.  Ideally, I'd really like to A/B it with someone else so I can do it blind - I'd just like to get a better idea of how the sound changes.  Assuming I get the wiring right so I don't screw anything up, is there any danger to my effects to hitching up a rotary switch with a couple batteries and maybe a powersupply, so I can get a quicker A/B between effects?  Is "hot switching" of power like that okay?  I assume I want to make sure my switch is "break before make"  so I don't send the wrong voltage to my pedal, and maybe make sure there's a pull-down resistor on my output to avoid some pops, but otherwise I should be okay?

[Paul Perry (Frostwave)]

That guy is not only rude, he's probably wrong most of the time as well (suprising how often rudeness & ignorance go together - but I digress!).
What you need, is the "flatdying battery simulator" or whatever it is called at geofex.
More here:http://www.geofex.com/Article_Folders/pedalbdpwr/pedalbd.htm

Why do high resistance batteries make gear sound different sometimes? Because giving a comon resistance to all teh stages (via the battery) can cause feedback between stages. Of course, another aspect of this is the presence of any filter caps from power to ground.. that will be trying to eliminate the effect. Nothing simple in the world of fx..