It seems like these are not very common - it is hard to find any reports of "juice jacking" in the wild other than demonstrations at the Defcon security conference and proofs of concept by security researchers.

Android phones by default disallow USB debugging, and also always prompt the user before the phone is made available to the "host" computer connecting by USB as a set of file drives. If these two attack vectors are blocked, and the phone is handling the USB port just for charging, there's little the malware can do to force itself on to the phone. I won't say the threat is zero, and there may be badly configured phones out there or those with bad software, but probably not something to worry about a lot. YMMV. I use power outlets if they're available, but if none are nearby and I need to charge, I'll use a USB slot without worrying about it.