Can an aspiring leader say "no" to work assignments and still move up at work? The answer is "yes"—if you choose your "no’s" carefully.
When interviewing executives for my book, DeeDee Wilson, Chief Financial Officer at Aritzia, a Canadian clothing company for women, emphasized,
"Generations X and Y look at work/life balance differently, and because of that it’s more acceptable these days to say ‘no’ to international projects or assignments with travel. If you do say ‘no,’ make sure your argument is well thought out—not emotional— and that you provide alternatives. Don’t say ‘no’ all the time. In my career, being willing to travel, to do things outside the norm, and to even be willing to live outside the USA have helped me progress—not just because I said ‘yes’ to the opportunities, but because I learned so much in those circumstances that I was better able to grow and add value."
Certainly, saying "no" is needed if you are overloaded beyond your capacity to do quality work. In those cases, give your boss compelling, rational reasons why you need to decline the opportunity.
I recommend coming up with alternatives if and when you say "no." One way of expressing this is to say something like, "I would love to be part of the internal taskforce you suggested, but given the demands of the Johnson and Myer accounts, I would need one of those clients to come off my plate before I could serve on the taskforce."
Phrasing an alternative, or a condition to you taking on a task, shows your boss that you want to do a good job but that your resources are not unlimited. Said Erin McGinnis, National Committee Chair of the Society of Women Engineers, "[Y]ou need a good reason [to say ‘no’]. If you say you can’t stay late, give a reason and be upfront. I think it’s important that if you have to say ‘I can’t make it work this time’ that you can counter that by offering another time when you can do it. Ask the question, ‘Can we create a way for me to still do this?’"