I recently watched a Clint Smith video where he discourages topping off your magazine. His reasoning is that you should carry the same number of rounds in every magazine and topping off gives you one extra round and can throw off your round count.
Counting rounds may work in practice or in a gun sport, but it doesn't work under real life conditions. In shooting investigations, it is extremely common, almost to the point of being a rule, that people under count the number of rounds they fired in an actual shooting incident. This is why one should never answer the question of "How many rounds did you fire?" If you say three, but you fired seven, you've made a statement that is at best inaccurate and at worst could be spun that you are lying about the incident.
Tom Givens will sometimes have the students gather around after a drill and ask them how many rounds are in their mag. I've rarely seen people get the number correct.
The reason for the top off is so that the firearm is at full capacity. Rounds equal time. The number of people who have successfully reloaded in the midst of a close range gunfight is extremely low.