I'm back for some more help. On my stairs (enclosed on both sides with wood - not drywall) I'll be installing 5/16" thick x 2-1/4" strips. Still debating on whether to start at the back or the front. Question for now is, do I need to leave a gap for expansion where the riser will meet the floor? My plan is to use 1/4" painted plywood for the risers as I removed the stair nose from the risers that were carpeted. If I don't need a gap, then it seems that it'd be easiest to start at the back and work forward once I've got the dry fit right. Thanks for all the great input on this site.

Ya, dry fit first and put the rip if necessary at the back, then work forward to the front edge, ending with a full board to interlock with the nosings. You are using nosings, right?
I would glue all pieces as well as nail. start with bottom riser, then the tread, and on upward, will mean no trim needed either under the nosing or at the junction of tread and riser. Gives a cleaner look, I think.

OK but what about needing a gap between the riser and the strip next to the riser? Is is needed or can I but right up to the riser? Thanks for the help.

By putting the strips in first, the riser will fit nicely on top of your new "tread" leaving a nice clean look to the assembly.