If I register a really good .com domain name, should I also register:
Which definitely, which maybe and which no? How do you know where to draw the line?
That depends very much on how important the site you are talking about is going to be. It is not unknown for spammers and such to buy those "alternative" domain names because people tend to make spelling mistakes and then land on those pages. Of course this will only happen if your site is really huge, otherwise nobody will bother.
Some slight variations so users have less difficulty remembering your site (hyphenated for example) or international TLDs if you have a site in several languages are reasonable extra domains to register.
First of all, when it comes to variations, you'll have access to all the subdomains that you could ever want, so choices of variations are at your fingertips already. e.g. i.areallygood.com iphone.areallygood.com, my.areallygood.com
If you have .com, that's the primary name to have control over.
Misspellings will only matter if you make the site worthwhile and frequently used, so certainly that's the place to focus first. You have to have a popularity like google.com before misspellings start to matter for you. You should have plenty of warning before your site approaches that level. If you're buying more than 2 domain names in the early stages of your site, you're doing it wrong.
So I think the only place to really watch yourself is alternate spellings. Ideally you'll create a domain name that is unambiguous so that you don't need to consider alternate spellings at all. The more clear your choice, the better your pr will be, the better your seo will be anyway. If you must create a site like abearlygood.com, then you'll have to consider getting abarelygood.com as well, but better to avoid the problem to begin with.
We had a domain name that ended in "live" and when the radio campaign went to air, a lot of people heard "live" as "life" (especially as the word "life" fitted our product as well).
We found after registering the "life" version of our domain our traffic increased dramatically.
