I'm thinking about maybe painting some or all of my sons 04' mach1 mustang. Is it necessary to remove the glass?

No, but it sure makes for a better job.

It is just soooo much easier to shoot without glass being in there. When I do kit builds usually something in the way of glass is messed up somewhere (most of the time it is a scarred up windshield) so I have a local glass guy come out and help me pull everything. When I'm done I have him back out for whatever new replacements I need and then install everything back.
You can do the "rope" thing which is okay but not preferred in my opinion. I take about 1/8 to 1/4 inch nylon rope and gently lift up the welting and cram it in so it stays up at an angle at least allowing for clean looking edges once the stripping is let back down. Obviously masking all the rest of the glass and welt face.

I'm with Darrell. I pay a guy to take out the windshield and rear glass and when done I pay him to put it back in again. That way if anything goes wrong he is responsible for it.

3m makes a special masking tape for lifting the edges of the window seals. It has as stiff edge that slides under the seal so you can lift it evenly and the other side is tape to hold it in place. I usually add another layer of masking tape just so it doesn't let go. The rope trick also works but like everything else it requires some effort to get it in the right place.

I remove and install my own Glass.
Removing Glass makes for a more professional looking job when all the edges are all painted the same with no Tape lines or sharp edges from Tape lines Plus you can replace the window Wipes and or Caterpillar felts while the glass is out, Repaint Pillar posts Package tray areas all easier with Glass removed. Removing and installing dash boards is easier with Glass removed as well.

Some newer cars 90s and newer have the seal Vulcanized to the Glass from the factory and removing the glass incorrectly destroys the seal. The seal in these cases is only available WITH New Glass, This can be a Problem and worth employing a Glazier for the job. They have some specialized tools made for removing Glass that are expensive.

I have had Glaziers refuse to reinstall Old removed Glass and or refuse to Guaranty that they wont break it while removing or reinstalling. This is why I learned to just do Glass myself after Paying some one else to Break my Glass a couple of times and Yes I have broke some over the years. But I have saved my self a lot of money over the years too.
If its an especially expensive Piece of Glass with a Vulcanized seal I may chose to have a Glazier do the work but Glass would have to cost North of $500

The Most expensive Piece of Glass I have ever installed was North of $20,000.00

Thank you for the replies. I've only shot one car, a 91' mustang coup. which had the moldings to remove, this newer stang doesn't have them. Makes it more difficult. Was just wondering what others do in this situation, thanks again