Handling
There are both pros and cons to handling, but care should always be taken to wash your hands thoroughly with anti-bacterial soap both before and after doing so to ensure that both you and your terrapin(s) are safe from cross-contamination.
Arguements against handling all revolve around it stressing the terrapin out, but if you are prepared to take things slowly (often VERY slowly) and "earn their trust", then it is perfectly possible to have a terrapin that doesn't mind being picked up, and will swim over to you and let you pick them up without struggling. This can be especially helpful if they are in an outdoor pond and you need to pick them up for some reason (health check or bring them inside), or if you ever need to take them to a vet.
They best way to get your terrapin(s) used to being handled is by starting with hand feeding. They will quickly associate you with providing food, and lose a lot of their fear of you. From there you can begin to carefully lift them up, give them a quick look over, and then put them carefully back. If you do this at the same time as giving them a weekly treat (such as prawns or dried shrimp) then this will once again make them associate you with something good.
The most important thing to remember is you're working against millions of years of instinct which say you are bigger and therefor a predator. If the terrapin is struggling, then they are stressed and you should leave them alone. As helpful as it can be to have them comfortable with handling, some terrapins never will be, even as adults, and you need to respect that.