If you're looking at this from a musician rather than singer/writer's point of view (or alternate between the two), I've found that it's often easiest on everyone to come up with the music first. There's nothing worse than languishing in the jails of the dictator songwriter (well, maybe burning alive. But nothing else
).
Not to say that this is always the case, though. Some of the bands I've been in were very cohesive, but the thing about them is that our writers had much broader ideas about the direction of the song than I normally would have expected- they didn't protest much when we took it upon ourselves to change in the original structure, or argued semantics. With those, we almost always started with the rhythm, though we did have one occasion when Geoff (the bassist) had been strumming his accoustic on vacation and brought back this slinky add6 progression that was just to sweet to change.
Thinking back, a lot of these songs had a very distinct jammy feel to them. The songs I remember most fondly were much more compact. They were also turds that took ages to squeeze out.
For example, the band I'm in right now has been writing the same song for a year and a half, and I think we've only just scratched the tits of the iceberg, to use that old cliché. Whenever I write songs and bring the words to the table, I get very defensive about any changements. That once lead to me getting the boot... But only once.
