Check the "rake" of your mast using the "plumb bob" method with a halyard. If you feel it could lean forward a bit without going past vertical you could put a few turns on the forestay to remove the slack from the backstay. That said.... fore and aft stays aren't as critical as the shrouds in keeping the mast straight. Many, many boats have adjustable backstay tensioners important to sail trim applications. On fractional rigs, where the forestay mast attachment is below full height, backstay tension is used to control mast bend, which is an important mainsail trim adjustment. For masthead rigs like Sirius 21, where the forestay attaches at the mast's full height, backstay tension is used to control forestay "sag".... which is an important headsail trim adjustment.