It depends. I actually feel that keeping at least moderate garrisons in my eastern provinces is fairly realistic regardless of whether the Mongols' arrival is imminent or not.

I say this because the Golden Horde were the not the only steppe peoples to attack out of the East. Ask the Byzantines, the people of Novgorod, the Kievans....they all dealt with barbarian tribes raiding and attacking throughout a lot of their history. Therefore it's not unreasonable that they would've maintained a certain military presence in order to discourage and/or drive off said invaders--long before the Mongols ever showed up on the scene. So in my opinion, I have a perfectly valid reason for stationing troops in my eastern lands.

That said, I try very hard to not go overboard. I don't make a habit of always deploying 20,000 men in and around Khazar, or anything like that. It's true that I often have a good-sized army just over in Georgia, but that's just because it's a "chokepoint" province (and would therefore have a powerful army stationed there anyway). As for any steppe provinces under my control, I station modest garrisons of 300-500 men, depending on my needs and capabilities.