Bouncing emails is pretty much a waste of time and bandwidth; the "From:" addresses are hardly ever real, so all you are doing is creating more traffic on the net - one for the bounce and another when it returns undeliverable. However, many believe forwarding the SPAM to certain sites that collect them may help. Dno't expect much decrease, however...
Whether your address is 'private' or not is almost irrelevant. There are programs that 'create' domains and addresses from thin air ( well almost ). So your 'private' domain is only guaranteed to keep real people from reaching you with it. Of course, if you respond in any way to the message ( see below ), you can be logged as a 'good/live' address and your address becomes very desirable to other SPAMmers!
If you're using Mail ( and most other email clients ), I don't think there is a way to 'preview' a message in order to see what's inside it. In other words, viewing any Mail email will automatically send requests for all/any graphics in it, even the 1 x 1 pixel, invisible gif's that you'll never see. Of course, you can turn graphics OFF, but I still think it's better to use a program that can read the messages without being able to download the images. I use PopMonitor to 'preview anything that might be in my 6 mailboxes. It is capable of deleting messages or flagging suspected SPAM with it's own set of filters. I don't use that function anymore, preferring to let SPAMsieve learn from the bad stuff.