If you mean the single player campaign missions then yes if you want to finish the campaign. Once you beat the campaign missions then you can do whatever you want. Eventually if you don't play the campaign missions and instead you just take jobs off the job board or people you will level up and then you have to go see Juni or whatever the objective says to do because the people and I believe the job board will say you already have a mission to do so it can't give you one. But also Juni or whoever there is will ask you to accept or decline the mission she wants you to do. I've never declined so I assume that if you do decline you can back to job board missions and etc... until you level up again. If I'm wrong feel free to slap me with the fish of your choice.

"You can pick your friends and you can pick your nose, but you cant pick your friend's nose."

Han Solo - "What an incredible new smell you've discovered [!"

Do Decline them, you have to go back to it at some stage though, sometimes straight away
Some funny smalltalk you haven't seen there either too...

tip - when juni says "TRENT, MEET ME ON MANHATTAN....SOMETHINGS HAPPENING!"

Ignore her and go about your business trading. Anytime you clear a waypoint a new one will instantly be shoved in your face to go to manhattan. When your satisfied you have money to burn you can buy the best ships/weapons/shields at any of the bases you stop at along the way on the missions through the storyline and elsewhere in the systems you pass through.
Before you go back to manhattan, get the 3 wrecks in the badlands and haul ass with the cardamine and your set!