Great question. This really depends upon the tax laws in your country but I can tell you what we do in Australia.
1. Domains purchased for less than $1000 we expense.
2. Domains purchased for greater than $1000 we capitalise and depreciate over 3 years.
3. Expense all registration costs.
We ended up going to some of the best tax lawyers in Australia and they came to the conclusion that domains slip through the cracks of the taxation system because they are NOT assets but expiring license rights.....then it went on and on.... We ended up applying the small business tax rules.
I hope that helps you out