In the US believe anything over 250g that you are flying officially needs to be registered. I don't think anyone is really checking but if you were to get in trouble or have an accident then it would probably come up if not registered. Also certain AMA fields may be more or less restrictive about checking for AMA membership and/or registration, I've never spoken with anyone outside the internet about it.
Registration mandatory from: 30 November 2019
The UK’s new Drone and Model Aircraft Registration and Education Service is now live.
There are two elements to the online system.
Anyone responsible for a drone or unmanned aircraft (including model aircraft) weighing between 250g and 20kg needs to register as an operator. The cost for this is £9 renewable annually.
Anyone flying a drone or unmanned aircraft (including model aircraft) weighing between 250g and 20kg must take and pass an online education package. This is free and renewable every three years.
Both of these requirements become law on 30 November 2019.