As long as you wait for 6 or more confirmations, your coins are safe. Waiting for 6 confirmations would make it quite difficult for Slush to pull ahead of the rest of the network with their own chain, and make it quite apparent if they did.
Even if Slush is really attempting double-spends, it really wouldn't make much sense to do so on a miner payout. It would make sense for many sorts of transactions (instant-payout gambling sites, digital or physical goods, etc.) but not to compensate you on work you already did for them (if they wanted to play that game, it'd be far easier to simply never pay you). All that would give them is miners that want to leave because they're being cheated.
A more likely double-spend scenario would be as described at the GHash.IO & double spending post on Reddit. In short, it is believed that GHash.IO executed double spends against a Bitcoin-based betting site.
It's also important to note that blockchain.info's information on double spends being attempted could very well be misinterpreted (e.g. maybe an orphaned block, with a later payout, would result in the warning you see). That address has a very long history.