If I create a wallet on one PC, can I log into another PC and use it from there?

Yes, Omniwallet stores your encrypted wallet on our server.
You can login/access your wallet from any computer with internet access as long as you have your WalletID and Password.
We still recommend that you backup/export the private key(s) for any addresses in your wallet.

When it comes to the recovery of Bitcoins/Mastercoins/Omni Protocol Properties the private key is the end all hold all for access.

As long as you have the addresses private key backed up someplace secure and safe you will always be able to import that into any Omni Protocol enabled wallet to access your funds.