Is it safe & secure to import my wallet into multiple Ethereum clients?
Generally speaking, yes. Ethereum clients are extremely good at handling key/wallet files. Most can handle a key file simply copied into the appropriate directory while the application is still running, with no issue. And as long as all of the clients are connected to the network, they will properly retrieve and update things like the account nonce without issue. That said, this doesn't mean importing your wallet into multiple places is a good idea.
Things to watch out for:
- Don't try to send transactions close to simultaneously from different clients. Leave some time for one client to learn about the other transaction and update the appropriate nonce.
- Security risks add across all locations where a key is stored. In other words, don't put your wallet in lots of places if you can't tightly control and protect all of them.
- Watch out for local state used by blockchain applications. Although it's not common yet, upcoming technologies such as payment channels and state channels depend on an updated local record of operations not contained on the blockchain. This could easily cause problems for you in the future.
All in all, there's very little reason to have multiple clients sharing the same key. Do yourself a favour and send balances between clients with a normal network transaction! It works 95% of the time, every time.