It is always important to warm up your muscles before you do any type of exercise, strength training and cardio. It gets the blood flowing to the muscles and 'warms' you up so you are less likely to pull a muscle or put you at risk for injury. I would recommend a 5-10 min warm up on the bike before doing any resistance training and a 5-10 min 'light' stretching session or 'light' warm-up on the piece of cardio equip you are using. You don't want to stretch too much because then you are putting yourself at a higher risk of injury. As far as after your workout, it is important to stretch as well, moreso than prior. Your body needs to cool down, loosen up, and recovery. Hope that helps.