Welcome to 3Reef, Calawah!!
IMO an RO/DI system may not be an absolute must, but not using one adds too many variables to an already fragile equasion. Having been without one for more than 20 years and fighting algae every step of the way; I have to say that now--I'll never again be without an RO/DI unit. The way I have mine "wired up" is using a hose bib adapter in my laundry room off a Y. Off the main water line is the Y. One end goes to my washing machine, the other is the inlet to the RO/DI system. My unit comes with an ASOV (auto. shut off valve) without it, when there isn't RO/DI water being produced, there's still waste water running down the drain--not good.
I got mine from airwaterice.com, who states they designed this ASOV about 6 years ago and have yet to have a problem with one, having sold thousands of them.
I purchased a float valve too, but I haven't yet drilled into my sump to install it. I simply plumbed the line out to my aquarium, and each night I turn a T valve, and let 'er rip for about 10 min's until about a gallon of topoff RO/DI water is produced. I let it stream into my sump, since it's a high flow area. I'm very happy with the arrangement. So are my fish and corals, apparently; although they haven't said so specifically.
To address some of your other questions; No, you don't have to have a holding tank on either end. I don't use one. You could get one, though--it would allow you have a few gallons on reserve and be able to dispense it as though it were coming out of your faucet. The downside is they're about $60 for the smallest unit, about 3.2 gallons.
Additionally, you could let your system rip all day into a 55 gal drum for water changes and the like, but a float valve installed into that drum (or bucket) would allow the event to be an unsupervised one.
Whatever you do, try to keep us posted with pictures. We love 'em!!
