When we draw scale diagrams, to determine resultant velocity etc, should we accurately draw it with a scale?

Yes, you need accuracy to draw a scale diagram. The word
"scale" represents the certain amount the original dimensions have been reduced or
enlarged.

The scale of a drawing is represented in this
way:

- 1:a, which means that all sizes from the drawing
have to be divided by the certain amount "a", to determine the real sizes, therefore the
scale has been reduced (e.g. 1:2 scale means that the real dimensions have been halved
in scale diagram).

- a:1, which means that all sizes from
the drawing have to be multiplied by the certain amount "a", to determine the real
sizes, therefore the scale has been enlarged (e.g. 2:1 scale means that the real
dimensions have been doubled in scale diagram).