^^^^^^^^^^^It makes no difference in terms of safety or harm to the gun or magazine. It WILL slow down your reloads, and it MAY cause a magazine to not seat properly.
There's your nice straight, short answer.
In addendum to that, when inserting a magazine...always give it a tug (down) to check that is has fully seated.
After charging your weapon, no 'press check' is necessary. The 'normal' arrangement in your magazine for traditional AR cartridges is 'staggered'. If you need to verify a round has chambered, simply note (before inserting the mag) which side the top cartridge was located.
Drop the mag and note where the current cartridge is. IF opposite the first, then a cartridge was stripped from the mag. and you are 'hot'. Reinsert mag, tug....you are good to go.
Disclaimer:
IF a magazine has been modified to use a center feed follower (usually a Big Bore or wildcat cartridge) this would not work for you.