What is the risk of using number in image style preset name (imagecache in D6)
I have never seen the term "alphanumeric" refer to anything other than a combination of letters and numbers.

Since the style name gets used in the URL, the restriction to "Use only lowercase alphanumeric characters, underscores (_), and hyphens (-)" likely has to do with avoiding any escaping issues with URLs and for portability across filesystems that may not support the full range of characters in filepaths.

Personally, I have never (nor have my clients) run into any issues with using numbers in image sytles, though as a best practice I lean towards using descriptive names for use, rather than size, in the sytle name.  So, I will try to use "thumbnail_square" instead of "thumbnail_90x90", in case I end up changing the size at some point.