Sitting in Michael's Lap
I will probably get a lot of grief for this, but I don't think that centering does anything for a poem except make it more difficult to read. If it's a greeting card, fine; otherwise just leave the darn thing left-justified so I don't have to hunt for the shorter lines ...
The obvious exception to this is visual poetry, or any other poetry that uses a unique spacing style to add to the content of the poem or emphasize some theme. For example, if I write a poem with a repeated couplet, I might choose to indent the couplet in order to enhance its effect in the flow of the poem ...
Hope this made sense ... LOL ... and no offense Dream, I know you like to center your stuff ... just my humble (oh, who am I kidding?) opinion ...
"Nunc lento sonitu dicunt, morierus"
(Now as I hear this bell tolling softly for another, it says to me, "Thou must die.")