You cant say no at the beginning because youn need the ship. After that you can say no. Shell just wait till you say yes. But you need to do the missions to gain levels so you have to do them eventually

Yes, most of the time it's okay to say NO. Sometimes she might give a bit of a verbal lashing like "<sarcasm> Oh thanks Trent. Just when I needed you </sarcasm>". But Ozu (that Blood Dragon Lieutenant) told me something like : "Yeah. I can see you need more time to prepare. Normally I wouldn't, but because of you .... blah blah blah"

And I got more time.... but after that mission .... you don't get the option (that's part of the storyline, not because you said NO).

excpet for the first mission, you can say no to the other missions, you just come back to see her at that place later to say yes

This is something I didn't learn till rather late in the game and wish I had known. I would just barely get a little money saved up when I would get her call. After I finally figured it out I ran trade runs until I saved up 1 mil then went to see her. It's a good thing I did too since I only had 200,000 left at the end.

And just for clarity, you can either decline her...or simply not go to visit her until you're good and ready. It seems to work either way. I'm 26 hours in the game and only in the middle of the third mission. What I've been doing is, after every mission, I go and explore a WHOLE system.

For example, after Mission 1 I explored the New York system in full. I landed at every single base (that I could with my current alliances) and scoped out the whole system, searched the various fields/clouds, talked to everyone, etc. Every time I landed, I read the info about the station (and various places uncovered in my map), I took a "random" mission, picked up some cargo (whatever seemed a good buy to hopefully sell at the next place I land), upgraded my ship, etc. I also read all the news at each place. It's making for a fantastic experience as there's quite a bit of exploring and such before I take the next "story" mission, and by the end of the game I'll have explored a lot of systems. (May even start doing more than one system before each story mission eventually).