Returning the whole object on an update would not seem very relevant, but I can hardly see why returning the whole object when it is created would be a bad practice in a normal use case. This would be useful at least to get the ID easily and to get the timestamps when relevant. This is actually the default behavior got when scaffolding with Rails.
I really do not see any advantage to returning only the ID and doing a GET request after, to get the data you could have got with your initial POST.
Anyway as long as your API is consistent I think that you should choose the pattern that fits your needs the best. There is not any correct way of how to build a REST API, imo.