It depends on the type of song and genre. Sometimes I write with a tempo in mind. Sometimes the lyrics and meter dictate the tempo...eg a slow ballad or an upbeat pop song. Some songs can be played with different arrangements and tempos. Compare my version of Scott Campbell's "Champion" to others on the challenge thread for an example of a song that can have many different arrangements and tempos. Sometimes when working a song the tempo has to be adjusted to find the optimum rate. A minus or plus 10% change can make the song sound completely different.