Personally I don't think it makes sense to build in redundancy if you're using Heroku, maybe a fallback to a simple offsite "we're down page" but that's it.

My reasoning is that if you are on Heroku you should be small and leveraging Heroku's management to the fullest. If you are big you are paying an incredible premium to be on Heroku (thought experiment: when does the premium start costing more than a full-time systems engineer). There is no Heroku equivalent from another company, at least nothing that's really turnkey in the same way, so if you build redundancy you're throwing away the benefit of Heroku's PaaS.

So in my mind if you need redundancy you need to be building your own system stack. Whether it's co-located, dedicated, or cloud-based is a separate issue because it's more or less commoditized. You build your system on a portable platform and then redundancy is just a proportional amount of extra work.