Don't count on it. Tail wags have lots of different meanings, some friendly and some not.
You have to look at the body posture of the animal and everything else it's doing. Is the dog growling or bearing its teeth? Is the fur standing up rigidly on its back?
How is the tail wagging? Stiffly, tucked and wagging, nice and flowing, is the whole body wagging? That all has to be taken into consideration.
Some signals to consider:
- Relaxed, comfortable dogs generally have a gently wagging tail held horizontally or slightly lowered.
- Confident dogs carry their tails up.
- Dogs with their tails down may be stalking prey, feeling cautious or indicating friendliness and respect to the one they're greeting.
- Together with a lowered head, direct stare, closed mouth, ears held back and eyes wide open, a lowered tail is a clear warning to back off.
- A tucked tail indicates anxiety, avoidance or caution.
The biggest lesson is, never approach a strange dog unless you are sure that it is friendly. It's just safer that way!