Sometimes I get an autocad floor plan from a designer to import into Sketchup. I can then take my post placements directly from that. But more often I'm designing a frame, at least a first concept, before the designer has a floor plan ready. In that case, I "snap chalklines on the deck" using the tape measure tool. If you double click on the red and green axes, you'll get construction lines aligned with those axes. Then drag out from those to set the rest of the grid (use the VCB to precisely position these). As far as adding labels to these, I agree with Mike, that SU is a little clumsy about it. I use the text tool, and double click on the grid line (at a non-intersection), and then type the label. This will give you leaderless text. That's what I did here:
The forum software shrank the picture. The see it full size,
go here. Here's a
link to the Sketchup Model 
