Does a mage need to maintain line of sight/touch in order to sustain a friendly manipulation spell?
I know you need to be in line of sight to initiate the spell, but I'm not so sure about sustaining it.

For example, could a mage throw an armor spell on an ally and then take cover?

What about harmful spells?

No, yes and no.
Line of Sight (or alternatively, touch or astral perception) is only needed when casting any spell.

it depends on how you define harmful spells. Combat Spells are instantaneous and as such need no sustaining anyways. Others are either sustained or permanent. Those two follow the normal rules for sustaining spells.
You only need to touch your victim once with decrease [attribute] or ignite for the spells to take effect.

In short, you don't need LoS to maintain a sustained spell, but wards, background counts, and counterspelling can all disrupt said spell, not to mention your sustained spell can be easily astrally tracked to you. Careful when maintaining the funny decrease charisma spell on that executive for days...it may backfire.