With beer brewing, the rule of thumb is the cooler and more consistent you can keep your fermentation the better! When yeast is forced to work at higher temperatures, it will produce a large amount of fermentation by-products, such as esters and fusel alcohols. These by-products are responsible for all sorts of weird flavours in beer, flavours that are not associated with good beer! Therefore the cooler and more consistent you can keep your fermentation, the less esters your yeast will produce and the cleaner more commercial taste you will achieve for your beer!
Most generic yeasts that come with your beer tin concentrates will usually specify fermenting between 20-25C, although this will give you your best results, these yeasts generally can handle fermenting warmer (up to 28-30C) with a lot less noticeable esters than if you were using specialty yeast. Again though, the recommendation would to be to stay as close to the ideal fermentation temperature as possible!
When purchasing and using specialty yeast, the esters caused by warmer fermentation is a lot more noticeable. The ideal fermentation temperature is usually 18C-20C for ALES (with a 2-3 week fermentation time). Whilst the ideal fermentation temperature is 13C-14C for LAGERS, (with a 3-4 week fermentation time).