I suggest IDing to the level you prefer. It is perfectly acceptable to ID only to the species level. It is my understanding that the Great Crested Grebe subspecies are determined primarily by range rather than visible characteristics; for such situations, I personally tend to ID only to the species level.
Subspecies can be a contentious topic and the same discussions tend to happen over and over each time a thread about subspecies is started. So anyone curious to know more about different stances on subspecies designations should search the many existing threads.
To name just a few:
Thoughts on Subspecies?
Subspecies. How useful are they?
Improve support / workflow for subspecies?
Question Regarding Subspecies Identification