Well, the maximum image size that can be uploaded to photomacrography.net is only 1024 pixels square, with file size 300 KB max.is there degradation from shooting in raw, changing to jpeg or tiff to stack in Zerene and then uploading to PM?
Those numbers are so small that it's inconceivable there would not be degradation from camera to uploading.
So you have to decide: is uploading to photomacrography.net your only goal? Or do you have other goals, and if so, what are their requirements?
For my own photography, the goal is always to get the highest quality image that I can, consistent with the amount of work that I'm willing to put into it. That means there are tradeoffs, which I have to think about for each situation.
For deep stacks shot in studio, where I can closely control the lighting and don't want to pay the costs of shooting raw, I shoot high quality JPEG.
For shorter stacks, especially in the field or when the lighting is especially challenging, I shoot raw. Usually I convert those to 16-bit TIFF, but if I plan to retain the Zerene Stacker project with converted images embedded in it, then sometimes I'll convert to JPEG because JPEGs are so much smaller than 16-bit TIFF.
In some cases, sure. If the lighting is challenging, then shooting in raw may give you more control that ends up producing a better result even at the reduced size required for posting. Or if your camera treats JPEGs harshly, typically by oversharpening, then actual-pixel crops may show better results from shooting TIFF. I have even seen cases where TIFF resolved a bit more detail than JPEG, though that varies a lot between cameras.IOW, is there any advantage to shooting in raw or tiff when uploading to PM?
In the end, I'm afraid there's no substitute for running your own tests and seeing what the tradeoffs are for your equipment and your goals.