Like cycomute said if you have a long unbroken distance of road riding it might be worth starting off with road tires then switching when you get to a long stretch of dirt. Again this is something you can play with before the trip so you know how the bike handles on the road and dirt under load. Just because a bike comes with one set of tires doesn't mean they're optimum for your use. Also you may get the on/off road versatility you want just by switching one tire.