[Radek]

The answer is difficult and, most likely, every W&W player gives you a different answer. so, there is my answer:

1. If you are only gaining by changing role then do it (almost) ASAP. Example: mage->warlock - change role immediately because the warlock inherits all mage capabilities and adds new ones: all magic schools are native for him and he gains Moon and Fiend magic. Another example: warrior->barbarian or rogue->barbarian - change role immediately because the barbarian is a warrior rogue, which combines all warrior and rogue capabilities. In both cases, you are losing no ability of your previous class.

2. Why only "almost". At low levels, you are gaining new levels quickly. Therefore, you can add a bit more spells in your spellbook, gain some training opportunities, and gain a few more skill and ability points by delaying the role change to about level 4 or 5 instead of changing role at level 1.

3. If you are losing some capabilities by changing role then try to minimize your loses first and then change role. The most important loss is the ability to learn new spells in the new role. Each role can divide magic schools into two groups: the native schools (the ones listed in your manual at the descriptions of the roles) and non-native schools (the rest of magic schools). You cannot learn new level 7 spells in a non-native school. Therefore, learn all level 7 spells in the school that becomes non-native after role change and then change role. At least, gain all level 7 spells, which you consider important. You will be able to gain lover level spells in a non-native magic school.
Note: the explanation above concerns learning new spells, not casting spells. If you already know the spell then you can cast it no matter what's your role, level or something else. A barbarian (no native magic schools) can cast level 7 spells from any magic schools even at character level 1 - if he already knows the spells.
Another exapmple: role specific traits. Some traits can be gained only by a particular role. If you want these traits then you must gain them first and then change role.