The tire companies might tell you something different and I'll wager that Four wheeler magazine would tell you something different from that. From my years of faithfully reading the magazines, especially Road & Track, my understanding is that there are several factors in mating tire width to rim width. Here are some that I think (IMO only) are relevant to an old Land-Rover:
The 5.5-inch rim has its extra 1/2" offset all to the outside (as it appears to me) so it makes the wider 235/85 affect steering lock less both for that reason and of course because the tire bulges out from the rim a little less. Even 7.50x16 tires on 5" rims require adjusting the steering stops to avoid rubbing. The 6.5" rims appear to also have all the extra width added to the outside so have much more offset. Josh discovered that to his horror on his Discovery, on the way to Mendo this year. As far as I know, the only 6" rims Land-Rover produced were on the Rostyle steel wheels that were used on the early Range Rovers. They won't fit on a Series Rover's stock hubs unless the centers of the wheels are machined out to accommodate the hubs. Classic Range Rover, Discovery I, and Defender alloys are all 7" wide.
A tire too wide for the rim width gives poorer transient response in handling (mainly an issue on the road at relatively high speeds), adding vagueness and slow response in entering and coming out of corners. Probably also adds to wandering tendencies that are otherwise present on the vehicle.
A tire too narrow results in sidewalls that are too vulnerable to rock and stick damage off-road. Rock crawlers usually (so I understand) use rims that are relatively narrow for the tire width for less danger to rim and sidewall damage, especially when the tires are aired down.
Non-optimum width relationship, whatever that might be, may increase chance of unseating a bead. However, I expect that as long as the tires seat properly when installed, this is mainly an issue when aired down. Oddly, a recent test by one of the 4x4 magazines found that rim guards (sort of an extra bead, on the sidewall, just above the rim) on tires, especially on the inside bead, actually increase the likelihood of popping a bead when rock crawling.
Also bear in mind that all the old Land-Rover rims have no safety bead and are intended for (and probably only safely used with) tube-type tires. If you want to use tubeless tires, safety beads need to be added. A machine shop can put each wheel on a lathe and weld in a safety bead. It's my understanding that most tire manufacturers recommend against putting inner tubes in tubeless-type tires.
The steel Defender wheels (LR# ANR1534PM) are 6 inches wide, the so called Wolf wheels (LR# ANR5593PM) are 6-1/2 inches wide and the Disco I steel wheels (LR# NTC5193PM) are 7 inches wide. All these fit a series rig.