That's about the difference you'll see due to temperature of the mash. Just like gravity, pH readings are temperature dependent. The "ideal" pH of 5.2 is at mash temp, which would make it 5.5 at room temp.
It's actually pretty neat. The offset more or less cancels out the temperature correction. So if you take a room temperature measurement and just read the strip directly you're actually reading the mash temp pH.
Anyway, what I generally do is check the pH with a ColorpHast strip (actually, half of a strip - they're plenty big enough to use each one twice) about 5 minutes after sealing up the mash tun. If it was off I'd use whatever salts I was holding back for the boil to correct it. So far I haven't had a mash that was far enough off to worry about though.