with most of my characters i almost always solo rank. Part of the reason is that I like to get my skills up and another part is that I rarely have time for long ranking sessions and have to drop link/quit out unexpectedly. Ive ranked up to 45 or so solo. The down side is that it takes a while and you may have to eat a few mob deaths in the process if you dont know what you are doing.

Especially for newbies, solo ranking will teach you what skills do, how they work, lag, damage, etc. A lot of this stuff you do not have time or the opportunity to observe them in combat very much.