Depends where you're going. I regularly travel around the Greek islands for 2/3 weeks on my own with no reservations. Always works out. Wouldn't do it in central Athens in midsummer though.

Gertie especially In Athens it is easy to find accommodation in the high season, as this is considered low season. In 2012 it will be even easier , since less tourists are expected this year, and hotels are not going to be booked to capacity. ( unfortunately )

Well - it depends on what matters to you. We have specific hotel requirements - would not be happy stopping at a random gite or B&B - and always reserve in advance to know we are going to get what we want and that it's within our budget (not low but we want a lot for our money).

When it may take a couple days at home to decide on a hotel (and find the best price) I wouldn;t want to have to do it on the fly.