It is your choice in this matter. Some customers have a unique org id for each env and some use the sub-org structure. Now keep in mind even with a sub org it will have a unique ORG id but you will be able to see all orgs under the parent.
For logging into multiple orgs you can use the same browser with a tab in normal mode and 1 tab in incognito which will allow you to login to each org.
If you cant use incognito then you can also use separate browsers.