If it's a site I don't care about and it doesn't let me log in via some form of OpenID or similar then I don't log in there. I have various sites with my real name or with this handle. I game (as much as I actually do) under another handle on both Steam and Origin.
For those who care, I'm Christopher E. Stith. My online handles and nicknames over the years include cestith (@cestith for those of you on Twitter), mr_mischief (obviously), mrmischief (not everyone allows underscores), Mr. Mischief (some places allow freeform usernames), mischief (but not the same mischief as here -- that's someone else), Pyro X (an old MUD and BBS nick), Lord Pyro (briefly, before I convinced the chivalry and heraldry-themed computer club they were going over the top), datawerx, groundhog (born on 2 February after all and this was my CB radio handle when my dad was big into that scene), and my Steam/Origin gaming nick pyrogroundhog (which is simply an amalgamation of Pyro X and groundhog).
For most everything that uses a nickname it's been some form of Mr. Mischief since 1993 or so. It's rare to find me without my middle initial when I'm using my real name, but it could happen here or there.
I almost always get chacham. But, sometimes, because of a failed earlier effort i have to choose something else. Then i question, chacham1 (just add a number), chacham2 (it's the second one), or add letters before it.
Then i realize i care too much. Doesn't answer the question, but at least adds some humor.:)
Many sites require an email-address as user-name, so I often don't have much choice.
I only can decide how important the site is, and if I want to get mails from it.
For real important sites (like my bank) I use my "premium" address, for temporary important sites
(eg for shopping, booking hotels etc.) I
use my main address, and for others I use an address I only check once or twice a month.
And then there are sites where I use mailinator....
With google, i can use email@example.com. The random part is usually the site name. This lets me know who uses what email and possibly to block. The only place i have seen the + sign barred is Quicken for Mac 2015 Beta 4. (Beta 3 allowed it.)
I also have my own domain. I use a subdomain, so email addresses are firstname.lastname@example.org. This works except when the site does not support subdomains, which is probably because somebody wrote his own non-rfc-compliant address validator. Sub-domain seem to be safe from brute force spam, as they send it to domains, not sub-domains.
Really, my preferred username has undergone somewhat of an evolution as time has passed. I used to be "blue_cowdawg" nearly everywhere until I ran into my first occurrence of that already being taken by someone (cad!). On AOL I am "red_cowdawg" but I don't use AOL for anything but chat. Starting a few years ago I began using "SaltyCodwawg" and on a couple of sites I am known as "Plankowner985".
A special note here: I have also began creating an "alpha mike foxtrot" folder for my wife and offspring listing all the online communities I belong to along with the passwords. This is a sealed document kept where my family knows were it is in case anything untimely happens to me. Good practice in my humble opinion.
Peter L. Berghold -- Unix Professional
Peter -at- Berghold -dot- Net; AOL IM redcowdawg Yahoo IM: blue_cowdawg
My choice was kind of arbitrary. I use one of a small set of nicks on most sites, and one of a smaller set on the occasional site I don't care much about but where I need an account for support, etc. -- the site for the brand of Bluetooth speakers I use, for instance.