Being able to consistantly alter ball speed in a controlled manner is a great skill that will give you an edge over the competition. If you are just throwing a wild shot, that is not a good thing. I used to remember the average ball speed to aim for a few years back was around 25-26k's. It was a happy medium between ball and pin deflection. It isnt so much a case these days, the highly agressive cores and coverstocks seem to maintain excellent carry at the pins, while still maintaining enough energy so they dont bounce off the pins at really low ball speeds (say around the 20 mark).
So can you control the difference in ball speed? and how do you do it? pulling through the shot or increasing your foot speed? (ie walking faster). I wouldnt pay any attention to what people tell you with reguards to what ball speed you should be bowling at. You can see the reaction your ball is making when it hits the pins, too much ball deflection and your ball speed is probably alittle low, if the pins are really flying, but not keeping low on the deck, might be an idea to try and decrease the pin deflection and keep the pins lower.
I use a combination of two things, faster tempo, higher backswing. I can usually differ the ball speed from around 20 - 36k's if needed. Im not a great bowler by any means, but being able to change ball speeds has allowed me to keep with an agressive ball, rather than changing to a urethane or hardcase all the time (maximising carry) when conditions were dry or if i had to play down the boards.