The way I determine Multi-Class options when I consider a MC character, I look at what type of campaign i'll be playing, and what I would consider my "optimum level" in that campaign.
For example, If I'm playing SF Society, I'll try to make the "sweet spot" of my Soldier/Mechanic build around the 5th to 7th level. If I am playing a home campaign where I know we will be hitting higher levels (16+) I might stick to a concentrated build.
You have to analyze what you want, and when you want it, before you can come up with any MC architecture.