Is is necessary to put sealant around where the bullet comes in contact with the brass or is it
OK to just drop a dab on the primer and let it go?

The military uses something called asphaltum to seal the casemouth. I have no idea what it is. There are several products out there to seal the primer. The name Roy's Primer Sealer comes to mind...

Is it necessary? Well? Do you plan to go to war? Military ammo may be subjected to some real abuse. Modern ammo has pretty water proof components. The old recommendation was "artists" lacquor on the primer to seal that end. It comes in aerosol. If you care to drip it around the junction between the bullet and the neck, might be enough. If you plan to "go to war" might not. No idea where to get the black asphaltum... If your ammo is going to be stored in conditions where you are happy to live, I would skip it all together. Your call. luck.

Asphaltum is available at art supply stores if you feel the need.

http://www.dickblick.com/zz452/05/

http://yarchive.net/gun/ammo/waterproof_ammo.html

http://www.handloadersbench.com/forum31/4814.html

The stuff you can buy as "primer sealer" is nothing but fingernail polish, available free from your wife/girlfriend/daughter and comes in a varity of colors. You can also put it around the case mouth after the bullet is seated.

It's your ammo and your time. I like to spend my free time shooting ammo, rather than waterproofing it.