Oh, a decent question, now this makes my day.
It really depends on how you're working, are you doing a candle spell or are the candles an ingredient in a much larger working? Are you Cursing, Protecting, Healing, Attracting?
I like to think of it like blowing a kiss, you're sending an intent out there, a focused gesture in the direction of your intended. I'm going to list a bunch of methods for handling candles and my personal interpretations on it.
Snuffing out , I see it as dropping the mic, covering it, smothering it having the 'final say' and pronouncing that it is the end of that. Or perhaps as a symbol of 'binding'
Water , depends on how much water, a little bit is for a slight 'refresh' and 'revitalize' a large deluge? Destroying or agressively banishing something.
Burning Out , only time I use this is if I want to keep the wax which will be infused with my intent, as a small charm to be added to a spell bag, or just kept like a magnet in my purse.
Blowing out , as I mentioned above, like blowing a kiss, or, a form of the 'evil eye' or pushing away, perhaps ending something, a direct, quick gesture.
Burn out fully, or burn for a little while?
You can reuse your candles, despite what people say, just remember to cleanse them or have certain candles for specific works, so like a big thick blue candle for Healing Magic, that you would light and pray over.
The only time I wouldn't reuse a candle for a different work is if I have inscribed names, sigils etc on it - because then it becomes bound to that intent or person - such as a white candle I have has names, sentences and sigils all over it, which I light if I need to give that someone a little more help.