Hi, I work for Autoglass (ahem, not name dropping )

Ideally yes, they do need sealing. Rubbers stretch and weaken over time, especially when you change a screen. To be fair, Escorts don't often leak, out of the 40 or 50 I've done I think only a 4 or 5 have come back leaking, but then, I usually seal every non-direct-glazing screen I do (direct glazing is when a screen is bonded to the body, non-direct is for all glass that isn't) just to be on the safe side.

We use a product called Arbomast, which is pretty nasty stuff to work with - it's so fucking messy!

Hard to get hold tho as it's only really available in the windscreen trade... If you do find your screen leaks, pop it into your nearest Autoglass and I'm sure a fitter there would seal it for you for pennies - it don't take long and it's not worth it to the company to book it as a proper job, someone will just do it for you...