I would never ingest rain water off the roof or let the dogs drink quantities of it.
Neither would I...until it was purified. It is quite easy to set up a small filtration setup and then use the various methods of purification (sunlight, chlorine, etc...) to render it potable. The reason I prefer this method over trying to get all of your potable water needs met from the beginning is two-fold: 1) the infrastructure is already in place in the event that the "fallout" of the disaster runs much longer than originally anticipated, and 2) if done properly, this method can save alot of time since it is kind of a set it and forget it purification setup (and can be done as needed) as opposed to trying to front-load all of the effort when there are so many other important tasks to take care of in preparation for a disaster.
But of course, YMMV....I personally just have a bunch of guns, ammunition, and camo paint...I figure that'll be enough to get me through any disaster that comes our way. 
- Alex
Rain water is great. You can easily use SODIS to treat your rain water as long as your roof and your gutters aren't holding much debris. Ever looked in your rain gutters? SODIS will kill any bad bugs, but it wont remove chemicals collected from the roof. Leptosporosis is the main concern here in Hawaii , it can get anywhere animals take a dump. Your roof is that place (birds, rats, parrots if you live in Wailuna, etc).
Nice thing about Hawaii is that it rains often and your roof is usually free from crap that collects. Bad thing is your gutters trap most of that crap and it sits there unless you ensure they are clean. I have found some strange shit in my gutters....dead geckos, birds, and once a greasy tool (no idea where that came from)
Most roof materials are made from petroleum based shingles and that stuff washes off after heating up, but it usually reaches a point of no return in that its done leaching out petrochemicals. Unless you have recently had your roof reshingled or are in question, consult your contractor to find out what you have above you. Always look for sand (or gravel) ...seems to be the biggest offender with shingled roofs in my experience. Sand sinks, so set your cistern up to draw from mid level or put a filter in it if you are paranoid.
I have a friend on the mainland who's entire household's water supply (5 people) comes from the roof alone. You would be surprised by the amount of water caught by one rainstorm on a flat surface. He live exclusively on this and has had his water tested VS the available public supply and his is much cleaner in comparison in the mineral and chemical department. This is consumed only after treatmeant VIA SODIS on the same roof! I have a design for a SODIS disenfection setup that will cost you a few bucks and lives on your roof. It is adjustable based on your view to the sun (not an issue here as long as you can see south).
He does not treat his bath, dishwasher, or clothes water. He uses a simple roof t and gutter to funnel water to his cisterns. He uses several drums that are painted black and some white and has never had an issue. He has been using this for about 10 years and hasn't paid a water bill since then. His power is strictly solar and has a generator, but can't recall the last time he fired it for other than making sure it was still operational, it runs on LP gas and Petrol as well.
While he is truly living off the grid in a 2500 square ft house in TX, he is very well armed (25+ guns and over 500 rounds for each) and retired. He is also well prepared and has been practicing food rotation for over 10 years (since Y2K).
Out of all the shit he has going on, I asked for the ONE thing he felt was necessary for survival and his answer was CLEAN POTABLE WATER..
Seems this is the worlds' elephant in the room.
I think at one point we were going to grab some stream water and SODIS treat it and either drink it or have it tested....