Depends on YOUR goals. I am trying to make it up to the instructor level, so I log dives religiously, accurately, and as fully as possible. I even write down important, funny, or interesting stuff my dive buddies do. I always do my dive tables (after the dive unless I really need to do them before hand, like multi deeper dives) This has given me a sense of security when it comes to knowing for real how deep I can go and for how long, I also use the Navy dive tables so I know how long and how deep I can really go.
It is a good practice to log your dives the best you can.
But, if you are just going to go diving, and don't want to become a dive master or instructor or anything like that, than I would just suggest that you log your dives and do the tables, etc. until you feel comfortable that you are knowledgable enough where logging your dives wont make you any smarter, then why bother, unless you just want a good record of what you have done, you never know, you might want it someday when you are old and can't remember your kids names, you can look at that and at least remember your dives.