Generally it's safe, just don't sleep in the tall grass, that's asking for wild Pokemon to poke around while you sleep. I usually kept one of my Pokemon out with me just in case, but I haven't had any bad problems. Well... Once I woke up with a wild Growlithe watching me.

[chuckles]

Only twice.

Both were accidental, of course. [yeah by his tone you know neither we're accidental, especially combined with his smirk]