system
A chat on subreddit /r/pcmasterrace
human
I've always used the spread method. Upon applying new paste, I've never seen any pattern that would indicate any air bubbles. 

But this is just anecdotal.
gpt
I edited to add a random plexiglass video above.  They show bubbles getting trapped the worst in the spread method.  You can only see them briefly before the plexiglass surface is wetted by the paste and looks "good" but they don't have a path to escape.  It's like trying to put on a screen protector or window tint.