Saying "No" is promoted to manage stress, prevent burn out, and other defensive values. Instead I want to make a positive case for saying No and share 3 thoughts.
1 The value of "No" comes from what you do instead. The question to ask is never "is X worth doing?". The question is instead "should I use my time/resources on X, or on Y?". If you say yes to X, what will you say No to? Mortals with finite time and resources only ever chose between things.
Some personal examples
I said No to the default curriculum in my University, took 1 extra semester to finish. I used the extra hours to record music albums and tour. I finished my degree with an 18/20 mark.
I said No to a PhD offer in the Netherlands, and went on to study at MIT instead.
I quit sub-optimal professional situations and managed to get a better job next.
A lot of luck and effort came into it, but saying "no" was a requirement in each case. When you say No, do something you want more instead.
2. A clear "No" is good. Life is full of ambiguity and uncertainty, being clear is a gift to everyone. It's painful to disappoint a friend or co-worker: but they'll be more disappointed if you commit and then back out or fail to deliver. What works for me, if I'm not sure about something, is to take time between being asked and giving back an answer. Be sure you are clear on all your Nos and Yeses
3. Saying No in a group leads to better decisions. Time and time in my career I saw people being quiet or saying yes in groups. But later, or when asked 1 on 1, they admit they should say no. Saying "No" in a group makes sure everyone can share their view and you're more likely to reach better decisions. However, remember that disagreeing is not a synonym to obstructing (sometimes just disagree and commit). Nos lead to better decisions.
Finally: Apply the same logic to "No" from others. If someone says no to something you ask, then they weren't the right person and they saved you the time and effort of pursuing something that would not work. Welcome dissent, wish them luck and support whatever they are saying "yes" to instead. Give others' Nos the same respect you expect for you.