'Try' rather than 'tree', as it were, is certainly acceptable - given the rhyme-word is 'eye' - if one's aim is to fulfil the sound-pattern as shown by every other pair of lines in the poem and one is reading it aloud to a young audience. Nevertheless, I would tend to say 'tree' myself without any forced stress on it...ie say the whole word as it is said in everyday ciscumstances.
This has been controversial ever since he wrote the poem. But most people would just go with the normal pronounciation of 'sim-it-ree' or 'sim-et-ree'. We did, at the teacher's direction at school.
Of course, you could do the exact opposite if you were teaching a class of children in Scotland. There, the word 'eye' is commonly pronounced 'ee' anyway! Consequently, the 'tree' pronunciation of 'symmetry' would work just fine.
Heathfield's response was not there when I started my second reply, so when I said "do the exact opposite" I meant re my own original offering, not his!