...taking it off gets you proper access to the nylon sliders which you should clean and also gets you full access to the rails so that you can thoroughly remove all the old grease.
You can probably get away with not taking it off, but I would remove the old/wrong grease thoroughly and would take it off. Really not that hard if you follow my pix on the cable reattachment, and you will learn how the cable works for the fateful day when you have to replace it (easy, BTW, but sounds tough).
Removing the decorative/protective strips - 5 minutes
Removing the internal fabric panel - 2 minutes
Popping the cable connector - 1 minute
Removing the two retaining bolts - 2 minutes
Removing the roof - 30 seconds
Times 2 I make that 21 minutes to R&R - good return on time IMHO as a) you know it's done right, b) you reduce stress on the motor and cable because off the old goop you have cleaned out, c) you will never incur the wrath of a bunch of brilliant engineers who spent hundreds of hours designing this, and d) you get the opportunity to adjust the roof if it is not dead level with the rest of the car's top.
Finally, your car is way too pretty for a ghetto fix.