Very much this. If I know I'm about to possibly consume a non-negligible amount of resources from a server by indexing a website with lots of subdomains, I typically send the webmaster an email asking them if this is fine, telling them what I use the data for and how often my crawler re-visits a site, asking if I should set any specific crawl delays or do it in particular times of day.
In every case I've done this, they have been fine with it. It goes down a lot better than just barging in and grabbing data. This also gives them a way of reaching me if something should go wrong. I'm not just some IP address grabbing thousands of documents, I'm a human person they can talk to.
If I'd get a no, then I would respect that too, not try to circumvent it like some entitled asshole. No means no.