Keeping the tempos the same is only necessary if you are looping. Even then you should employ techniques such as stretching and pitch changing in order to make your sample sound better over the beat. You can't let the sample dictate the beat for you. I usually never look at the tempo of the sample because I don't loop. It's important to chop a sample according to the sounds. You need to chop it at the beginning of the beat, or before a hit or chord strikes. If you just randomly put a chop in the middle of a chord it will sound wrong. I guess it's kind of hard to explain but most people should get what I'm trying to say.