I don't have the option of having a sub come while I do my F&P assessment, and those are the only formal assessments that we do for reading.
What I do is make my Daily 5 a Teacher's choice day, and my class knows that means I will be allowing them a bit extra during Read to Self, and they cheer when they find out.
I make sure I take my "little wiggler" to test before he has too much sitting still time, and then when I continue on day two I make sure he has very good fit books, a great choice place to sit- for him that is inside of our "little wiggler" pup tent. We use a pup tent to block his view of the other students and their view of him. The door is open towards my table so I can see what he is up to, and this was my students idea.
I get a lot from the F&P assessment and not only about miscues. Both the fluency and comprehension pieces are really important to my students because they are all ESL and will often be excellent at decoding the word, but blast through a passage without paying any mind to punctuation or phrasing. As a result they might have no miscues at all, but have no clue what it means.
I think its very important and guiding to my practice to be able to test my kids myself. I use the information I gain to help them with their goal setting, and my own planning about who needs more 1-1 time or to be checked in on more often.
Great question!