Tumble is the closest thing to a must have skill, due to the AC boost it provides. Unfortunately melee warrior types, who tend to need it the most, can only take it as a cross-class skill unless they multi-class, but it is worth investing in even so.
Discipline unfortunately tends to be a questionable investment. Since a discipline check is a skill check against a DC equal to the attack roll triggering it (with occasionally a few other special modifiers added in), if you have a good Armor Class (and you really want to have good AC here) any attack roll high enough to hit in the first place will more than likely also be good enough to beat the Discipline check. This tends to make the skill rather redundant most of the time. It is not completely useless, since it can occasionally help you against Natural 20 hits that trigger the skill, and in addition, in Swordflight, there are occasional special scripted checks against the skill. As a rule, though, there are probably better things to spend skill points on.
Having one of the three conversational skills at a high level will often be helpful and as Persuade is a class skill for Paladins it makes sense to invest in that. Taunt is another paladin class skill that might be worth investing in. It can make many fights much easier.
Throughout the series, you will sometimes have access to henchmen with rogue skills and sometimes not. If you take such skills yourself you can certainly expect to get some use out of them, but if taking them as cross-class skills you probably cannot get them to a high enough level to get all that much use out of them. There are some benefits to having them, but it is also quite possible to manage without them (which could be said of almost all the skills in the game, actually).
