There isn't a right or wrong of it, though I believe that if we took a vote on this site, there would be some preponderance in favor of measuring without pressing. Some people will accept a measurement obtained by pressing in as legitimate, on the ground that it shows how much length you have available for use; others will not, on the ground that such a method does not measure what you actually show. A disadvantage, on this site at least, of using pressure to determine your length is that if the figure that you get does not match what people see in your photos, they may think that you are a liar. On the other hand, not many guys, if any, can resist the temptation to adjust their posture, angle, and so on to get a more favorable measurement of themselves.
One thing you can always do is to take both measurements and split the difference.