are you talking lagering or fermenting a lager? 'lagering' is the process of chilling a lager down near freezing for an extended period (4-6 weeks or more) after primary fermentation is complete... if you're talking fermenting, you'll need temps ~50 deg F pretty consistently, a few degree swing probably won't make much of a difference, but if the fridge gets down to 38 degrees or something periodically, it'll put the yeast asleep and likely won't allow the beer ferment fully... you can put a thermometer in the fridge, set it on one of the higher settings and see what the swing is. if you can keep it in the upper 40s, the temperature of fermentation will probably keep it where you need it. of course a temperature controller would be your best bet with a fridge. you could also set it in a cooler/bin of some sort with ice/water in it to maintain a lager fermentation temp. it might end up a little high but i know a few people who have made decent lagers without having it at exactly 50 the whole time...