RE: Cycling - hamstrings
Cyclists do often suffer from back pain and 'shortened' hams, correct stretching and strength training can remedy/prevent this.
That said we are talking guys who do a lot of cycling, if you are using it for general fitness/cardio purposes along with other forms of exercise and weight lifting then I would not worry in the slightest.
I have always cycled, my main sport was running and it never hindered me - in fact it allowed me to get more training in without the impact/injury risks running more would carry. I always stretched after my running training as well as on my rest/recovery days and I have been told my hams are actually over-extended now.