This quote is deep in a thread in "General..." and I thought dedicated "Project Finishing" readers should benefit from it too so I'm quoting Tom King here.
"With single pane glass, if the sash or door is painted on the inside I use exterior paint on the interior side. Water will condense on the glass at some point and interior paint will buckle when it runs down. I've never come across painted on the interior sash with single pane glass that didn't have paint issues on the horizontal pieces inside. Never had to redo a sash with exterior paint on the inside."