Just like each line of a poem is weighted in importance (last words most important, first words next, middle the least), the poem itself is weighted, but is weighted differently in importance.
What I mean by “weighted” is how “good”–to put it plainly–and interesting that part of the poem (the word, in the instance of the line) needs to be. You don’t want your reader to give up on your poem. You want them interested to see what happens next. If they get bored and stop reading, the poem is likely a fail.
So, to keep a line interesting and unexpected, you might want to break it at a word that is unusual or carries extra meaning or can be taken multiple ways. You might want to break it where it does not break naturally syntactically. You might also want to avoid a very simple rhyme scheme, one which a reader can anticipate and is therefore too bored to continue reading.
The first line is the most important in a poem. The title needs to be good enough to catch attention but does not necessarily carry the importance of the first line–this is seen in the many poems of very generic titles written over the years, that dazzle despite their modest titles (Elizabeth Bishop’s The Fish immediately comes to my mind). However a bad title can be particularly off-putting or cause the poem to not be taken seriously.
The first line needs to draw the reader in, set up the poem’s tone, and be interesting syntactically and lyrically. The middle of the poem is of course also important, and just like structuring a paragraph in essay writing, the first line of each stanza and last line of each stanza will hold a little more weight than the middle lines.
The ending lines are the next in level of importance. I’m always studying how poets end poems. I am a fan of some rhyme or repetition or divergent statement at the end, but there are many ways to give that sense of closure. I like to leave the reader with an image to think of or a single line to linger in their minds–something that may cause them to return to the poem. Because though the poem is written for you, it is almost moreso written for your reader.