I have done it both ways. The first three times, it was with the windshield in. It was a PITA but I did it. The hardest part is putting the dash back in, because it is pretty hard to line up the screws back in their holes when you can't see them. But every time I eventually got them in. I found that a flexible screwdriver worked pretty good, but so did a short handled small-headed Phillips screwdriver. Either way just takes a bit of patience.
More recently, I removed and re-installed the dash with the windshield out. MUCH easier! But would I remove the windshield JUST to take the dash out? No way. Too much chance of breaking the windshield, and always a chance that it won't seal as well afterwards. But if I had a good reason to remove the windshield, such as re-painting the car or the gasket is weathered, or the windshield needs replacing, I would do it in a moment.
When you do remove the dash, make sure that you do EVERYTHING you need to before you reinstall it. Such as re-cover it (a vinyl kit is available from OGTS that looks great; John Warga just did his, with the windshield IN). Also make sure that the heater core is in good shape. They can be re-cored by most competent radiator shops. Go through the fan motor. Buy new brushes and make sure that the bearings are well lubed. And most importantly, when you have the heater out, make sure that the inner fender (over the fan, has a flat ledge from inside the wheel well) is intact and not rusted through. That's a very common reason for the fan to quit; mud builds up on the ledge, it rusts through, and then mud drops down onto the fan motor.
HTH