turret join seems?

This is where getting hold of some good references can really help out. Most of the vehicles that you have mentioned have lots of pics on the net itself, a great referrence source. Some seams are "good" seams that should be there while others ... well ..... aren't. Check the references of the vehicles that you are building but pay special attention to the details of the seams. Some will need a little touch up, some will need nothing at all and others will need major surgery.

I sanded a bunch of parts for a 2cm Flack gun. When I saw the real thing it HAD those seams in its construction. Well its not like I'm putting the seams back into the model, but I spend more time eliminating ejector marks rather than seams in many situations.

well, it depends on the date of the model, and the manufactuer. modern tamiya, DML, and trumpeter, and various more are for the most part, acurate. if u are thinking of weld seems, then yes. the manufactures do add those. but there are also joint seems that show where to parts have been joined togeather, the show up as lines. they should not be there. but like i said, if u are buildng newer kits, you shouldnt have to worry about that. as for reference, if you are not sure about a seam, just go to your hobby shop; bring a piece of paper and a pencil and write down where the main seams are or if any seams should be removed. but for the most part, you should worry more about adding seam that arent there, not taking them away.