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In my opinion, it is much more beneficial learning a high level language than a specific architecture assembler. But I fancied learning some ARM assembler just for fun since I know some 386 assembler. The idea is not to become a master but understand some of the details of what happens underneath.
You will see that my explanations do not aim at being very thorough when describing the architecture. I will try to be pragmatic.
ARM is a 32-bit architecture that has a simple goal in mind: flexibility. While this is great for integrators (as they have a lot of freedom when designing their hardware) it is not so good for system developers which have to cope with the differences in the ARM hardware. So in this text I will assume that everything is done on a Raspberry Pi Model B running Raspbian (the one with 2 USB ports and 512 MB of RAM).
Some parts will be ARM-generic but others will be Raspberry Pi specific. I will not make a distinction. The ARM website has a lot of documentation. Use it!