It's much more conventional practice to make y the subject, I'm not sure myself if there is some rule but here are my arguments for putting y as subject:
*If you get a complicated high order differential equation like:
It's going to be very hard to verify that
is a solution without transposing first.
*y in terms of x will indeed be a function, but it is quite likely that x is not a function of y (e.g:
)
*I think by SOLVING it means that which FUNCTION has the property that it's 4th derivative with respect to x multiplied by x-1.... equals 0.
Although I agree that if the question does not mention SOLVE but isntead says "find the time at which the particle is at x=3" then I do endorse cutting down the computations as it is still answering all the the quesiton requests.
I'm not sure on any of this though hah.