Where a new drywall meets an already finished/plastered/painted ceiling or wall (at a corner) is it good enough to simply apply a cost of plaster all the way to the corner or is it necessary to use paper-tape and jointing compound across to the adjacent surface (Eg to avoid cracks)?
It's always better to add paper or mesh tape at seams in walls or ceilings. You can use plaster instead of joint compound, mud, but mud is easier to work with. In corners, it's good to use a preform interior or exterior corner. Some don't need to be taped because they come with tape installed on both sides of the corner strip. Check your home stores for the types they carry.