Bike touring isn't quite like backpacking. On normal days you pass stores, gas stations, restaurants, parks, etc. Therefore, you can replenish items daily. You also have the ability to hitchhike in the event of a serious problem. I carry stuff that I'm likely to need or use, and don't carry too much "just-in-case" stuff.

Getting water is seldom a problem. You can fill up in bathrooms, at the soda fountain at Taco Bell, etc. So I carry three water bottles on my frame, but usually only fill two, unless I have to go through a longer stretch with no services. However, if my route takes me through long stretches with no services and really hot weather, I've also carried a Camelbak for extra water capacity.

I carry tools for fixing flats and fixing broken spokes, and a few extra bolts for ones that might vibrate loose (plus a small tube of Loctite), plus a good multitool with a chain tool, but that's about it. I only carry one spare tube and no spare tire. If my tires look like they're getting worn I replace them before the tour. If I was going through an area with lots of thorns I might bring more tubes, but in many years and many tours I've never needed more than one spare. Usually the tube with the hole becomes my spare (with a patch on the hole) if need be, but if I've installed my new spare, I stop at the next bike shop and buy a replacement (and jetison the tube with the hole. It may not be ecological, but you do what you must on tour.)

Take as little as possible, but don't short-change yourself either. I'd rather carry a little extra weight and be comfortable than live an overly Spartan existence on tour. I'm doing it to enjoy myself, after all. After someone's done a few tours they usually settle in on an understanding of what works for them (and remember, what works for me may be unsuitable for you; we all have our own priorities.) If you find yourself with too much stuff, stop at a post office, buy a box, and send the extras home. I've done this twice, and I've read about lots of others doing the same. Just make sure you don't sent too much stuff home. I did that once, and I missed some of the things during the rest of the tour.

As far as loading goes, I try and load my front wheel quite a bit, since broken spokes on the front are virtually unknown and I've broken spokes on the rear. I put small, heavy items in front (stove, gas bottle, pots and pans, tools, book, etc.) In the back I put clothes. I strap my tent and sleeping bag across the back. I have a Big Agnes air mattress. it packs up into a small package that fits perfectly on the platform of my front rack, so that's where it goes.

Remember, however, that loading up the front really affects handling. In order to stay balanced we usually make a series of small adjustments with our handlebars - usually so subtle and small that we aren't consciously aware of doing it. However, all that mass in front resists being put in motion (Newton's 2nd Law) and it becomes much more difficult to balance, especially at very slow speeds - which are often necessary going uphill, since we're climbing with such a load. If you're focused it's not that hard, but if you take one hand off the bars to wipe your nose, or try and take a drink, you may find yourself off the shoulder or wandering into traffic. My feeling is that this might be exacerbated if your load is unbalanced side to side. Furthermore, I used to have a whippy touring bike that had a terrible shimmy when it got going fast. I found that this was lessened when the load was carefully balanced. I always pay attention to side-to-side balance and do the best I can.