On one hand, them not doing it (now or in the immediate future) keeps the manga fluffy and wholesome.
On other hand, them doing it before March comes opens up SO MANY plot possibilities.
And on the third hand, man, i've just binge read this Series and it's so fyckung heartrending, jesus christ... They should totally engage. Buy some matching rings, say the vows, then tell their parents. Do ANYTHING to make their olds aware of their love.