Ask HN: Fast vs. perfect, semantic front-end code for MVP?

Just wondering where web designers/front-end developers draw the line between quickly prototyping code for their startup (perhaps using something like Twitter's Bootstrap framework), and having long-lasting, semantic code that won't need to be written in the short term?

This is my first startup and I'm finding it difficult to know when work is 'good enough' rather than obsessing on the details as is customary when freelancing.