How much tweaking of figure design (not content) can I postpone until the final figures are requested? I have finished all of my manuscript revisions except for some minor details in the figure. 

As I understand it, once the manuscript is accepted, a final draft of high resolution figures will be requested. In the past, I have also had requests to relabel and rearrange subplots.

Does this mean that I can wait until the final draft figures are requested to make minor tweaks, like aligning and sizing fonts and graphical elements? The content will be the same, but I still want to finalize elements such as the font sizes, line widths, title placement and legends.

In the interest of speeding up the process, is it acceptable for to submit revisions with the figures still in draft form?
As a referee, if figures are in a rough shape, I will absolutely call the paper out on that, and demand improvements. This will potentially set the acceptance back by weeks. Unless the figures will take weeks to get right—and in this day, that would usually imply  that more data needs to be collected—then it's better to just do it right the first time.