I did wonder if putting them on the lead at first sign of another dog gives my dog the impression there is something to be fearful of?
It's unlikely, especially if you work to make going back on a lead a fun activity with lots of praise and rewards for doing so. If seeing another dog, means going on a lead and going on a lead is a fun activity then the chain of events is unlikely to trigger any fear.
Also being off lead gives my dog the opportunity to run away and stops lead reactivity?
Think about distance here. If you clip onto a lead at a fair distance from the other dog and then maintain that distance, there is nothing to react to, iyswim. Lead reactivity tends to be
a) because the other dog is closer than yours is comfortable with, but the lead prevents yours from increasing the distance
OR
b) because the other dog looks interesting and is close enough to be tempting but not close enough to be able to greet, so your dog wants to greet but the lead prevents them, thus they get frustrated
OR
c) a seemingly weird combiniation of the two
Greater distance helps in all of those scenarios. It also helps the other owner if their dog is reactive, which in turn helps you because your dog is less lilely to be on the receiving end of any barking etc.
Ideally you want your dog to ignore most others dogs (because they seem boring) and only engage with friendly, well rounded and reasonably well behaved dogs they can learn about doggy interaction from. Which means you being selective on their behalf 