If you're adding "reddit" to the search query that's a bit different, in that case you're explicitly looking for results on reddit. Penalizing reddit in the rankings shouldn't change this use case.

I was more thinking about generic search queries that have reddit in the results. In my experience that doesn't happen very often.

I just don't think reddit gets a significant amount of traffic from search results so penalizing it in the rankings won't have a big effect.