I asked a friend to do me the favour of casting a critical eye over my latest manuscript before I submit it. One thing she queried was my practice of not capitalising the initial letter of every line of my poems. Evidently she is more comfortable with the convention of initial capitals.
For the sake of others who may be interested in this question, here is my reply to her:
Many poets still use the convention of capitalising the first letter of every line. At least as many, if not more, no longer do that. There's an interesting discussion of the matter here, amongst poets.
The practice of initial capitalising in English poetry began in the 16th Century. This changed with the advent of free verse in the 20th Century, as initial capitals would have been intrusive to the flow and to the various ways that poetry can now be arranged on the page. It is very common now for formal poets, too, to dispense with initial capitals, though some retain them. On the other hand, some practitioners of free verse, when using a fairly conventional arrangement of lines on the page, like to adopt initial capitals — but have to abandon them when they venture into things like shape poetry or prose poetry.
I write mosty free verse, but like to play with form sometimes. I don't want to be inconsistent within my own work so I adopt prose rules for capitalisation, whatever kind of verse I'm writing.
When all's said and done, these days it depends on the personal preference of the poet.