@The_Meta: Well since your Kindle will most likely arrive with firmware that has new UI (introduced with fw 5.7.x) you might as well update it to the latest firmware version (after jailbreaking and installing hotfix).
In my experience one of the most common reasons some people (including me) didn't want to update their Kindle past fw 5.6.5 (that was the last firmware with older UI) was that UI redesign.
As for reading the jailbreak thread, you'll need to read it once more since obviously you didn't get all the info you have to understand
As in, you can't leave you Kindle on "initial firmware" you've used to jailbreak. At the minimum after jailbreaking you MUST install the same build number (say "5.7.4") of "consumer firmware". Or it can be newer build. You only can't install older firmware build of "consumer" (= bin downloaded from Amazon site) firmware.
Initial/factory bin files are the only exception to that rule (rule being "can't downgrade firmware using built in procedure").
So: downgrade to initial firmware (to be able to jailbreak)>jailbreak> install "hotfix" (ensures jaibreak survival on next firmware install/update)> install "consumer" version of you choice (can be any between 5.7.4. and the newest/5.8.7.x)
Prior to discovery and "capture" of those initial bin files if you wanted to downgrade from firmware newer then 5.4.x you only choices were soldering wires on serial pads on Kindle motherboard or downgrade from diagnostics mode (if you gone to trouble to jailbreak and install "rescue pack" & "coward rescue pack" prior to updating you Kindle past fw 5.4.x; those enable you to boot into diagnostics mode using just USB cable).
Also, it seems to me that you've failed to understand that once you've installed "jailbreak bridge" (by installing the "hotfix" package = set of scripts that restore jailbreak certificate after firmware update) jailbreak should (and did survive practically every firmware update thus far) survive installing firmware updates (
as long as you don't factory reset and firmware update one after another!!.
So once you've done
complete jailbreak procedure you're free to update you Kindle to fw 5.8.7 and future updates.
(smart
) People tend to wait until somebody else writes here on MobileRead to confirm that jailbreak did indeed survive the newest firmware update when it arrives and only then do the update. Especially people that use unofficial/hidden functionality like "FONT_RAMP" thing (used for redefining those predefined text sizes in Aa menu) tend to wait for confirmation that Amazon did't remove it in the newest firmware update.
Since Kindle doesn't ask you if you want to download and apply newest OTA update but goes ahead and updates you Kindle while you're not using it, if you're one of those people that want to use bunch of hacks and mods for the lifetime of you Kindle you have to either keep you Kindle in Airplane mode 99% of the time or you must read about
current OTA blocking method known to work on
you particular Kindle firmware (keep in mind that "backdoor lock" KUAL extension doesn't work !!)... and only update manually (by downloading the bin file from Amazon site) when and if you decide its safe to do so.