Lycra -
I brought this up a while ago on the training forum, chiefly because I had the same "unhelpful" (shall we say?) comment about my unsuitable forefoot footstrike.
As noted there, I think it's right to say that most Elites run as you do (midfoot or forefoot rather than heelstrike), though a small percentage land on their heels first.
The only thing to be aware of is that forefoot strike can contribute more to injury (supposedly), and in particular if you land excessively on your toes this is not a good thing as it can bring on "Achilles" (!) tendonitis.
You might find as Dalya comments, that when you're running really slow, you're landing more on your heels - and I believe this is natural and desirable.
It surprises me that the subject is not more discussed - and it's oddly worrying to get such a comment from a running shop who you think would know better. Mine was from a common or garden physio who didn't really know about running, so he could be excused -though quite frankly at the rate I was paying him, I don't feel like excusing him at all!!!!
best. s.