well seeing as this post is in the the beer making area, i can only assume we are talking about fermenting beer....as tony stated, it depends on the yeasts, but that also depends on the style of beer you are trying to make...ale styles, and yeasts, are fine at room temperature....lagers on the other hand, in order to achieve the proper characteristics of the related lager beer styles, are supposed to be fermented at cooler temps...not fermentating at the recommended temperatures for specific beer styles and yeast strains can lead to off flavors for that particular beer type, and in some instances, may not allow the proper flavors to develop...when and if you are submitting your beer for judging in specific categories therefore, these off flavors and/or missing characteristics, are considered "faults", lending your beer to be marked down because of it...