Should startup founders sell their common shares to future or current investors,and when to do that?

I am really curious about this topic, and I am researching as much as I can. I want to start a tech startup, and I am just getting familiar with how the startup and vc world actually works. I came to articles and blog posts about founders taking money off table when fundraising, or pre-ipo or when there is huge round of investment from VC's. What is your opinion of this? Is this good thing to do? I am not asking for a very early or young startup, but successful one. Should founders ask the board or future investors for this? Without alarming them or sending red flags.