The correct grammatical way is to have the accents. However, the non-accented versions are probably going to get more type-in traffic.
If you do a general search on Google with a Greek keyword, it will return results for both the accented and non-accented work.
If you do your search such as ... ++"keyword in here" - then it will search for exactly the keyword/s you types in and include the accent in the search.
For example -
Google search for ξενοδοχεια ( Hotels ) without the accent
Search term - ++"ξενοδοχεια"
http://www.google.gr/search?hl=el&q=...83%CE%B7&meta=
Results - 437.000
Google search for ξενοδοχεία ( Hotels ) with the accent
Search term - ++"ξενοδοχεία"
http://www.google.gr/search?hl=el&q=...83%CE%B7&meta=
Results - 3.390.000
To check for which version is better, I prefer to use Yahoo. If you do a search in Yahoo for a non-accented Greek word, you will find a lot less number of results that you would for an accented Greek keyword.
If the domain you want to reg is available both with and without the accent, it would be wise to register both versions.
Hope this is of some help.
Alex