A Dallas newspaper article has noted that California and Ohio have different opinions about the legality of a search of a cell phone incident to arrest.
I thought the deputy quoted at the start of the article had good advice:
A recent California Supreme Court decision says police do not need search warrants to examine the cell phones of those under arrest. But local judges and a deputy chief for the Dallas Police Department say officers should obtain warrants before reading the contents of cell phones. Until the privacy issue is decided by the U.S. Supreme Court, "it's safest for us to go ahead and get a warrant," said Dallas police Deputy Chief Craig Miller. He said Dallas officers generally ask suspects for permission to search their phones and, if denied, obtain a warrant.
This matches the advice I used to give to officers when I was a prosecutor or advising law enforcement. Of course, this advice was not always followed . . .