When tuning an acoustic guitar, I do not tune the open strings. I instead tune the Octaves. Since most acoustic guitars do not have intonation adjustment, yet are susceptible to minor warping, I have found you can tune your acoustic, yet your octaves may be slightly off. But if you tune your octaves, if your guitar is slightly out of intonation, this will at least average the difference. Another alternative (besides tuning using harmonics) is to find the A note on all your strings and tune all strings to A. Now you are ensured that everything is relative to 440 hz.
A Guitar, regardless of age or price is constructed of physical materials.
Various materials expand or contract at different temperatures, moisture levels, atmospheric pressures, phases of the moon, etc.,etc.
You can tune your guitar, throw it in the case, go to a gig, whip it out and within minutes watch it slowly go out of tune simply because you are now in a hot, moist, smoky room under intense lights. Instead of a dry, air conditioned apartment.
As a guitarist, you should be aware of your tuning at all times. If you like to pitch bend a lot, you will stretch your strings a bit and should be able to compensate. Watch Jimi Hendrix tune by ear in mid note without missing a beat. Pure genius!
There is a direct ratio between your level of education and how long it takes you to tune up. Paul Simon, a college grad in music can tune a guitar by ear in 8 seconds. You can circumvent this by taking a course in perfect pitch ear training. Yes, perfect pitch can be learned. Just like speed reading. You just have to have that inner desire to be the best of the best and apply yourself 100% to mastering your instrument ;)
Ironically, 99.9% of all guitars are not technically accurate to a tempered scale in the first place! You wanna have you mind blown? If you are interested in accurate pitch, look into True Temperament Guitars: