I had always been taught that you should put two spaces between sentences. I'm not really sure why. I guess it is supposed to separate them more. It's a simple force of habit now to put two spaces after a period and I was surprised to find out, once I started making webpages, that even if you type to spaces, HTML will only render one.

I was also rather annoyed for a while because Lotus Word Pro, the word processing software I was using, had in it's grammar rules that only one space should go between sentences. I turned that rule off.

So, in answer to your question about whether it's a cultural thing, I don't know.