There are no "safe" herbicides, just as there are no "safe" pesticides. Whatever you do spray to control any "weeds" will have an adverse affect on your environment, and as long as you are going to plunk down more than a couple of inches of some material that will prevent these plants from getting access to the sunlight they need for growth there is no real good reason to use them.
Yes, there are a few of these "weeds" that might grow up through a pretty thick mulch layer, one I've found is the thistle. But any of these "weeds" that do grow up through the mulch are easily pulled.

Thanks....that's what I needed to know. That the amount of soil/mulch I'll be putting down will pretty much 'kill' weeds/grass(due to lack of sunlight).

And I appreciate that you mentioned 'mulch'...as I had forgotten to factor IT into the height of my bed (in relation to how deep I'd planned to plant - didn't want to have to dig - just fill with soil).

Looks like I'm gonna need third level of deck posts around the top. DARNIT!!! And I thought I was all done with that project :o)