Well, the thing is designing it lower generally means it doesn't look as good, some people don't have the software to view these things and the bandwidth is generally effected much on your side (like maybe 10x the bandwidth for 1 visitor, meaning you can only take 10% of the visitors you could with html, for example)
Issues change with each website, if you expect low traffic, by all means go ahead and try it. Another thing might be to make a HTML/image website and a flash website and compare how big each one is, then Just stick the smallest one on there.
It's totally up to you how you go about it. It's more of a trial-and-error when starting a website of any kind using any program to build it