While I whole heartedly agree that one should always play in tune, I'm surprised by the need to tune a guitar every day. It's certainly not in my experience.

I have a 22-year-old Martin D35 and use Elixer phosphor bronze strings. I practice six days a week. [u]One[/u] or [u]two[/u] strings may need returning every other week or so but that's it. I check the tuning once a week with a Snark tuner and the individual strings hardly ever move from dead-on, in tune. When a string does register out of tune according to the Snark, it seldom is great enough for my ears to hear it.

When not in use, the guitar is kept in a case with a fairly steady humidity a bit above Martin's recommendations. The room's temperature is also stable subject to gradual seasonal swings. Strings are changed by the time they hit 100 hours. I don't play gently but I'm far from a hard strummer. All that should help with stable tuning but I think the quality of a guitar and strings are the biggest factors with tuning issues, maintenance excluded.

Or, maybe I'm just lucky.