Check the line at the top of the comments box and if it says 

"Both critical and non-critical comments are welcomed."

Then it is fair game as far as i am concerned (more so if they tick the "critical only" box).  If someone gets shirty about a critical comment just politely remind them to check the box reserved for those who just want an "ooo excellent photo hugs xxx", and dont indulge their comment collection outlook next time as your effort is wasted on them.

Problem is that most people fall into (whether consciously or not - who doesnt like to see 20+ comments on a photo?) the latter category or at least bordering on on it - critical comments are a rarety on this site (and most other sites) exactly for that reason - people are sociable and dont want to offend others.

Personally I try to leave constructive comments (whether positive or negative) where (and increasingly when these days) i can.  Just do what you feel is correct and you will be fine - i'm my own worst critic so i doubt any critical comments (if based in reason (ie not "I think you needed to be closer to the lion to improve this photo" or "the corner of the building needs to be moved a bit to the left to help the composition"))  would offend me. 

"In every colour, there's the light.
In every stone sleeps a crystal.
Remember the Shaman, when he used to say:
Man is the dream of the Dolphin"

Rich Meadows Photography