Ultra-picky typography question

I'm working on a document that's full of measurements like 10' x 20'.  I've made sure to use prime marks for all the feet or inches throughout.  Am wondering if the x in 10 x 20 should be changed to a multiplication sign?  Is that standard when you're writing 10 x 20 or should it really be an x?

Hope that made sense.

Thanks, Phyllis