Hi, Stephen. This problem is definitely a headache for officers and raid leaders. You need to use a fair system that doesn't gimp your raids. However, it's always better to have too many than too few!
For a casual guild, you don't want to overcomplicate this process. The DKP
or Suicide Kings
approaches that you suggest are a bit much.
I'm also not a big fan of the "first signed, first slotted" approach. It definitely favors those with flexible schedules, and it gives an unfair advantage to the people who are online when the calendar items go up. Not to mention, this system can be "exploited" to a certain degree by guild members calling/texting/twittering/IM'ing friends when they see the events become available. You can wind up with the same people every time.
It's also an unrealistic system. What are the odds that the first 10 people who sign up will fill out the required roles and collectively have adequate gear and skill to succeed? You'll have to compromise no matter what.
My guild has been having this same problem for our 10s as well, and I'll share the way I've been handling it. It's similar to the rolling approach you suggest.
First, I invite everyone who signed up to the raid and see what we've got. I talk to my officers about the roles we need to fill and which roles have too many players.
We announce which players are guaranteed a slot, usually because they play a tanking or healing role with the right amount of players. If we ever have a large amount of new or undergeared players, we might designate a few veteran raiders for slots as well. Then everyone else rolls for a slot. The lowest rollers sit out that night.
However, anyone who has to sit is guaranteed a slot for the next raid they can run. That allows players with low-flexibility schedules to raid at a bare minimum half of the times they can make, if they somehow go on a cold streak with the dice. And if they really do have such epically lousy rolls, you can always step in and give them a slot anyway.
On a side note, I roll with everyone else. If I lose, I sit out. One of our newer players expressed surprise at this: "You're gonna bench yourself for your own raid?" I'm fortunate in that another officer is happy to lead the raids in my absence. This, to me, is just. If I'm going to ask others to subject themselves to this system, then I shouldn't exempt myself or any other officer from dealing with it.
It does suck leaving things to chance, so we only use this system for farmed content. For hard mode runs, we handpick our raiders, taking into account attendance, gear, experience with a particular fight, communication skills, class balance, etc.
Unfortunately, this surge in attendance that fall brings won't last forever. Finals, winter holidays, or sheer boredom will drive people away eventually. With Patch 3.3
on the horizon, though, it will probably last until beating down Arthas is as ho-hum as Coliseum dailies.
Rolling for slots has worked out great for us so far. If anyone else has come up with a good system now that fall has brought so many raiders back online, share it below!