absolutely not a hard rule. but if you are an established pro with a good referral rate that has your site well placed and advertised I would totally go ahead and have fun with the blog. Post what you had for lunch or why cilantro is a terrible addition to the taco.
But, if you are trying to generate traffic and interest it helps a prospect out when googling to find their venue or other vendors mentioned in your blog.
I would say having a blog post saying you shot a wedding at "insert venue here" and really enjoyed the "insert vendor here" as opposed to
"I photographed Melanie and ate great chicken surprise."
Of course that is a little extreme but you get my point.
I will even provide links to other vendors.
That makes sense.
But then I suppose if you are getting consistent clients there's not need to put anything other than weddings in your blog. I unfortunately am not one of them!