It depends on the update: a major update may indeed introduce a number of breaking changes with regards to custom style rules (and customizations generally), however minor updates should not cause any considerable disruption.
For example, just as styles targeting 2.0.10 would have been extant for 2.0.11, a rule targeting the layouts generated by Events Calendar PRO 3.0.4 would work pretty seamlessly with 3.0.5 and I’d expect them to still be valid when 3.1 arrives.
Further ahead is harder to predict, but (and this is nothing more than speculation) if we reach a 3.5 release or move to a 4.0 release you might again expect to have to re-tool any customizations if you want to use the latest codebase.
I hope that answers your questions 🙂