The link in comments provides a very good answer regarding fretting notes but when dealing with chords other issues start to emerge. Take an open A chord played with 3 fingers or an open Em with 2 fingers. Our anatomy does not allow most of us to comfortably place every finger right up against the fret so we put them on a slight diagonal to accommodate the different lengths of our fingers and the way our hands address the neck.
In the case of barre chords, our anatomy has a say in how we place the index finger on the board. Of course being as close to the fret is ideal because it requires the least amount of pressure and bends the strings less but some may find that their index finger cannot line up parallel to the fret and still place the other fingers to complete the chord. Others may find they can have their index finger perfectly parallel to the fret and right up against it. Others can achieve parallel but have to give themselves a little distance from the fret in order to be able to place the other fingers.
Bottom line, go with what sounds and works best and is most comfortable for you. Its ok if you can’t put part or all of your finger up against the fret. Just make sure you use just enough pressure to get the notes to sound cleanly but not so much that the notes become sharp. Experiment, look in a mirror, play the notes of a chord one at a time to see if all the notes are clean, use a tuner to see if you’re pulling or excessively pressing any notes to make them sharp. Nothing is etched in stone, use trial and error to see how you can get your best sound comfortably and with the least amount of effort based on the size, shape and flexibility of your fretting hand.