When i taught multi-eng students i always told them 5* is the max allowable for certification standards (as explained above), but i always added that the best bank angle that gives optimum performance varies with each aircraft (if i recall correctly, the seminole is 2.8* or something close - been a while, its in the AFM). In practice, it matters little if you fly wings level or try to maintain that ~3* that seems best for most twins, but if you fly 5* or the 0* on climbout, you'll be suffering the same performance penalty as the optimal * is in the middle.
Another argument for not neglecting that bank angle in favor of an 'easier wings-level climbout' is to maintain positive control of the aircraft. I had one particular student who constantly would relieve control pressure when he looked down to check the checklist or do something. Aircraft would begin to roll on him everytime.