I've done more than a few cars BOTH ways. if there might be rust under the windshield, yes it should come out. new England car or west car??? then on most models, the whole dash can easily be take out. then you can work on the instrument cluster/wiring e asily for sure.
taking out the windshield , depending on model, can result in ya having to buy a new gasket. $$$$$. can you remove and install the windshield or have to pay the glass guy?? how much time do you want the car apart. how long does it take you to DO the stuff under the dash with it out?
is it to be a "show poodle" or a driver?
like removing ALL the glass to paint. yep the resto will do that as they charge ya $80 / hr????? for me, taking the qtr glass in and out is always a major PITA !?
when it comes to some jobs on these old cars, what result are you looking for??? ans, are you results oriented or " anal retentive"?? ha