Many people do place value on correct grammar and usage. The importance is based on the individual. While I teach English grammar and attempt to use standard grammar in both speaking and writing, I am very tolerant of the language spoken by others. Most English speakers welcome even the beginning attempts of new students.

受过教育的人重语法；之收过很少或未收过正规教导的对语法不一丝不苟。

It's rather important, but people will be tolerant as long as you are understandable and can speak basic English.