If it's a story dungeon or trial, you can do it completely blind. People who do it when it releases have no choice but to do it blind anyway and they are usually fine.

In fact, when you have done them on release day you start to realise that the guides over-analysed it - spending minutes explaining a mechanic that lasts a split second and can even be ignored in practice. I don't blame the guides for that, they are supposed to do this to give you a thorough understanding of that (non-threatening) mechanic, so the answer is it's up to you if you watch it, but it's not necessary.

For high-end versions like (Extreme) and (Savage), I recommend watching a guide when matching with random people. They are usually unprepared enough and struggle to get anywhere as it is, so not having watched a guide will hinder progress even further. If the party all agree it can be done blind in the description, then that's acceptable and you can find blind parties occasionally.

Personally, I do story content blind, and I watch a guide for high-end content if I feel that, based on the normal version, it will be necessary. After playing for long enough I can mostly predict what the high-end version will be like though, so sometimes I just jump in blind and figure it all out immediately from experience. I did this for a lot of Shadowbringers extremes.

Whether I watch a guide really depends on if I feel like not watching one will hold the party back much.