I don't about how much difference it will make to your SEO, but it can make a huge difference to your visibility and appeal if you manually tune your posts as appropriate for the site. The biggest issue here is Twitter ... because it is such a different format to other sites, you really want to optimise your tweets for 140 characters. There's one organisation that I follow on Twitter that really hasn't got the hang of this – their tweets are just autoposted from Facebook (which is a bad idea in itself, because it then encourages people to only follow them on one or the other rather than both), which means they are written without any thought of the character limit, and then a shortened URL to the FB post if it goes over. So invariably you either get a brief message that is truncated by just a couple of words, meaning you have to load the FB page just for the sake of a couple of words that could easily have been squeezed in – or you get a longer and meatier post that hasn't been front-loaded, so you get 100 characters in and still don't know what it's really about, so you don't know whether you need to click on the FB link or not.
Manually composing your tweets is vital if you want to show real engagement with your community. That's possibly less of an issue on other sites where you are not quite so restricted in how much you can include, but even so I would rather see someone making the effort to individualise the posts rather than just an autoposter.