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.