I write in 4/4, but sometimes the initial riffs are 6/8 or some other signature, but I simply add a filler lick to bring it back to 4/4, depending on how it fits into the song. I usually carry around a metronome in my guitar case and when I write a rough riff, I use the metronome to determine timing and BPM, and to get the riff down real tight.
