(1) Explain your problem, don't simply post "This isn't working". What were you doing when you faced the problem? What have you tried to resolve - did you look for a solution using "Search" ? Has it happened just once or several times?
(2) It's also good to get feedback when a solution is found, return to the original post to explain how it was resolved so that more people can also use the results.
There generally isn't an issue with whether you press F9 in the middle of update or after actualizing all the activities.
Personally i press F9 after updating each task to see how the progress of said task impacts the schedule in its entirety. In this way I am able to easily spot out of sequence updates which may be erroneous or a result of a change in strategy / methodology. As long as all the tasks are updated as per the current status date or data date at the end of the day, there shouldn't be an issue.
The only concern here would be if you have resources loaded and the activity type involves the following types:
In such a circumstance, you would need to be wary of how your change of data date would impact the units allocated as a change in duration would result in a change in units.