A recent disagreement between partners in a recent start-up was on how many variants of domain names should be registered early on.
My view was that just the main domain (.com and .co.uk as we are in uk) would be enough right at the beginning as we really didn't have anything to protect yet.
Another view was that we should secure .net and variants with hyphens and variant spellings and plurals as well as singulars and all combinations of the above which would have increased the costs dramatically - probably by an order of magnitude.
Although I agree with Julie that domain names are relatively cheap, I've never had a problem with not owning variants.
The thing is, people just don't type arbitrary URLs in the browser window. And, if your company isn't trivially small, Google/Bing/Yahoo will be smart enough to present your website as the correct match for any search-like thing people do.
Finally, at least in the USA there's trademark violation if people own your domain. So for example at Smart Bear I just got smartbear.com, and if anyone tried to get something else AND was using it in a way that made it look like us, I could do something about it.
But definitely get the domain before the rest of the business paperwork or someone could pinch your domain. That happened to me!
Domain name variants should be the least of your concerns. Register the main ones (.com, .org, .net) and forget about the rest.
In the early phases of your startup, you'll be lucky if anyone understands your idea, let alone thinks it's awesome enough to squat similar domain names!
I slightly disagree with what's been said so far. Type in gogole.com in the address bar and see what happens. I think if you are serious about your project, and if you want to get startup capital, at the very least spend USD 100 in domain names. As a percentage of your yearly operation cost, it's really nothing.
There are common misspellings that you can research for your particular name. You can get very creative on this research, for example, in my case, I polled all my cofounders, asking them to rapidly type the name of our startup 200 times and buying the top 5 mistakes. Or you could try http://www.selfseo.com/domain_typo_generator.php for instance, or a similar tool.
Hope this helps
It might also depend on the type of product/service your going to provide.
If it were mass-market (push strategy), I would think you'd want to have most of the variants/mis-spells as possible to make sure you capture the great majority of people looking for you.
However if your catering to a niche (and probably via a pull strategy) then they probably know you before they find you and less variants should be fine.
The other thing to remember is that too many variants can dilute your brand somewhat. You want people using the right name/spelling from day 1. If you cater to all of the ways they may mis-interpret your name, you might be stuck making sure they can always find you through those variants.
If they are available at the time you decide on your main domain (".com" in most cases), I'd suggest getting the key variations at that time:
The extra domains will cost you < $25 a year if they're unregistered.
I would not worry too much about other arcane variations (and definitely don't worry about getting the .net/.org variations of those variations).
Domains are cheap.
We held a view similar to yours in the early days and as a result lost an important version of our domain (hyphenated) as as well as the .net variant.
So I guess it depends on what you are willing to risk. If I could do things over - especially when the name will be central to the business - I would register the hyphenated variants as well as the main ones (.net, etc.).