Tuning question

Hmmmm.....

Like many relative newcomers to guitar I thought I had this issue reasonably sorted but now I'm not so sure :roll:

Whenever I pick up my guitar I slowly pick each individual string and then play a few open chords. All the while listening carefully to see if I can tell which are out of tune. Only when I've tried to correct this by ear do I check my work with a digital tuner. My judgement is getting slowly better but I still feel more confident with the tuner in my gig bag. That, until this morning was pretty much the extent of my involvement with the issue of tuning.

I was happy!

This morning I forgot to turn the tuner off when I was warming up and I noticed that many of the notes I was playing were sharp. Okay, I did a little reading and now I realise that the very act of fretting a note tightens the string and puts it under more tension. Making it sharp. In some cases up to 20 percentiles sharp :cry: The effect seems to be more pronounced on the bass strings the nearer I get to the nut. I do try and play with as light a touch as possible in the left hand but suspect that I still probably 'over fret' my chords.
I'm not sure I can see myself developing the skills to vary the pressure in my fretting fingers to allow for the different string thicknesses/tensions. Always seems to be something new to learn :evil:

So, is this a tuning issue or is it a technique issue? Or should i just quit worrying and get on with learning Ziggy Stardust :lol:

Simon