I am quite often out of blank pages but there's always somewhere that can be stamped/overstamped or even stickered over if some country's officials care to do that. While I've never had airlines make a stink out of it, I have had some country's officials either annoyed by it or entertained by it. When I can, I still default to getting additional sections of pages in my passport in so much as a given passport can handle it. [Got another section of pages in my most recent ordinary "extra-pages-at-start" US passport just last week and this is probably it for this passport before it falls apart.] I always do this outside of the US because I don't like the idea of paying for additional pages.