It used to be the case that people were unsure if the logic pins were 5V tolerant or not due to ambiguity in the datasheet and the consensus was to be cautious, but it has since been officially confirmed that they are tolerant. Some other devices (e.g. relays, some MOSFET gates) might require a 5V output level to trigger, in which case you will need a level shifter/transistor, but 5V inputs to the pins are fine (and most digital devices that work at 5V logic levels will be fine with 3.3V logical high due to the way TTL levels are defined).
See discussion here: https://hackaday.com/2016/07/28/ask-hackaday-is-the-esp8266-... and official reply on Facebook (of all places): https://www.facebook.com/groups/1499045113679103/permalink/1...
> Teo Swee Ann i can reply officially here: it is 5V tolerant at the IO. while the supply voltage is at 3.3V.