I think that Marty's advice was pretty good, Bill, as long as the tray was left to dry well and limit rusting from water trapped in angle iron crannies. But I'd probably be too lazy or forgetful or procrastinatory to get it done monthly. I don't get much corrosion around my connections because I keep all exposed conducting metals coated with spray-on anti-corrosion paint; most auto parts and some hardware places carry various brands. NAPA's dries faster, but Permatex's is thinner and seems to penetrate into copper strands and nuts and bolts better - downside is its stronger vapors and maybe days to dry. But it gets in to places like Keith is talking about.
The bigger issue for me has been accessory metals. The hold-down straps' buckles and their tray mount brackets disintegrate, even when coated with the paint. The original plastic snap buckles melted within the first year. I found chromed cast metal ones at a farm store and they were eaten away within 2-3 years; ditto for the second set. I think acidic battery gasses travel down the nylon straps and concentrate on the metal parts. Amazingly, they melt buckles of any material, but not the strapping.
This year, most of the strap mount brackets to the tray finally rusted out via their screws, so I lengthened the straps enough to put them through the unused slots in the tray that seemed originally meant for them, in lieu of the metal brackets. I wrapped them around clevis pins on the outside of the tray and back through the slots to new plastic buckles; the clevis pins lay flat against the tray side when the strapping is tight, so the straps can't pull back through the slots. If they corrode, the clevis pins are easier to replace than the factory hard-to-find brackets that require screws. Because I had to take the batteries out to get the old strap mounts out, drilling out shot screws, etc., I took the opportunity to thoroughly clean the tray for the first time in 9 years. It wasn't as bad as I'd expected; after cleaning and a little wire brush and Dremel work, I touched up any bad spots with POR-15 paint, and attended to any other issues I saw.
I think I'll try rinsing at least several times a year when I check the 6 volts' water level, and maybe stave off another plastic buckle meltdown. I usually rinse the battery tops when done with water levels, but I could be more thorough with the job and wash down the strapping good. Thanks Marty and Bill.
-Joel