It depends on how I'm feeling and how my fluency is that day/in that moment. If neither is feeling great, or if I don't feel like working through a block, I'll substitute a word. But if I'm feeling better I'll just relax and find my way through it, whereas when I was younger I would just push harder against the resistance of the block. My goal - that I do not always live up to - is to never substitute a word to avoid a block. If I'm patient and present enough, I can work through pretty much any block; so substituting (or feigning ignorance, etc.) feels like taking the easy way out. Keep in mind, I don't think everyone should hold themselves to that standard.