As scJazz said, it's up to you.
For myself, I only allow myself to use other recruitment methods outside the ATB Personnel market when I'm on a 'friendly' planet and not on a contract. By friendly I mean either belonging to the faction I most recently worked for, a Hiring Hall planet or a planet belonging to a faction I consider myself on good terms with (worked for several times without working against them.)
But that's just my personal rule, you decide what's fun for you, but, from experience I will say it's more fun to be consistent with your own rules. So if you decide your rules are if you BUY a vehicle you can "Hire Full Complement" but NOT if you scavenge one, stick to that rule, try not to bend your own rules 'just this once' as it really takes the fun out of it.
On the same note, slavishly following the 'official' rules even if you don't agree with them is no more fun. They are a great starting point, but you should feel free to change them to suit yourself without feeling like your 'cheating.'