Beware that generally speaking a powerbank with a stated output of 5V will not necessarily always run at 5V, particularly when there is a sudden spike in current draw.
I believe their circuitry is intended to be antithetical to supplying sudden surges in current, as an active device such as a pi requires. Instead they are designed for recharging other batteries, which is a very different use case in which surges of current are undesirable.
I've had limited success with using various powerbanks to power various models of pi. In general, they work for far less time than their stated mAh would imply; one reason for this I have seen mentioned online is that this is a figured derived directly from the batteries inside, which are not 5V, and no adjustment is made to the number. In other words the fact they they are sold, e.g., as "5V, 12000 mAh" vs. "12000 mAh at 5V" is significant.1 And indeed, I have a decent quality 12000 mAh 5V battey pack with 4 x 3000 mAh 3.7 V batteries inside.
The 3.7 V is of course stepped up, but this would mean the bank is only good for 3.7 / 5 * 12000 = 8880 mAh @ 5V. In practice it will power an active (say 20-50% CPU) single core B+ for 6-7 hours, much less that an idealized 24 hours I've seen some people hypothesize online (including here).
Further, powerbanks do not seem up to snuff for a Pi 3. Even using a bank rated at 3A, the power led will flicker at boot and during other periods of stress.
If you are thinking of buying a powerbank expressly for this purpose I would recommend you look into something else. If you already have one and want to use, it can't hurt, but be prepared for disappointment in terms of how long it will last.
1. This might be construed as sort of reasonable since their most widely applied use is in charging mobile devices, which take 5V inputs but also use 3.4 - 3.7 V batteries. So if you have a 12000 mAh battery that is "good for recharging your phone 3-4 times", this is probably true if the phone use a 3-4000 mAh 3.7 V battery. Which is very common.
Of course, this also explains why they don't always operate at 5V -- they don't have to. They have to operate at say 3.5 - 5V, which is fine for recharging but not for powering active electronics.