Assuming I have a private key written down in old "uncompressed" format, does it matter whether I send BTC to the compressed or uncompressed public address? Funds sent to either are still controllable by the single uncompressed private key?
Or must I be sure only to send funds to the "uncompressed" public address, otherwise I lose the funds?
First, let's be absolutely clear that nobody without the private key will have any idea the two addresses are connected to the same EC secret. So, aside from the owner of the private key(s), they are as different as any other two distinct public addresses.
For the owner of the private key(s), they are (or should be) the same. It is my contention that a wallet that doesn't recognize both public addresses is deficient. [But I suspect most wallets are deficient, by my definition.]
As kjj explained, you haven't lost the funds. But you may have to convert the private key from one form to another and then import the other form of private key to gain access to those funds.
Here's an experiment: Send a tiny amount of coin to the
uncompressed brain wallet for "correct mule battery horse", which I gave a few posts up. Send a different tiny amount of coin to the
compressed brain wallet for "correct mule battery horse", which is given above. Add one private key at a time to your favorite wallet. See which transaction(s) show up in your wallet. Do not send more than you want to lose, because anybody who reads this thread may transfer the coins out of the brain wallet.
Once these transactions are in the blockchain, everyone who reads this thread could add one private key at a time to their favorite wallet and they can see for themselves which transactions appear in their wallet.