YOU DONT HAVE TO!
Its YOUR convention that YOU choose to answer your question,
e.g. let vertical velocity be v = u + at
If I choose a = -g then v = u - gt where +ve motion is upward, however,
Where I to choose +ve motion as downwards (why not) then
v = u + gt
(remembering that a falling body here would have +ve velocity)
You may encounter both conventions in your studies, and if it is not stated,
it is assumed that down is negative, but you could choose either to suit your problem.
Markers usually are aware of this and you should never be penalized as long as you assert the convention you use.