Any key works with localhost. So if you are only using localhost as development/test environment, then you will be fine.
Developer license and above allows unlimited domain names, so if you plan to get one of those, there shouldn't be any problem to use multiple hostnames. Standard license does not support more than one domain names, but it does support more than one host names --- as soon as they belong to the same domain name. Typically you can set it up this way:
Development servers: dev1.your_domain_name.com, dev2.your_domain_name.com....etc
Test Servers: beta.your_domain_name.com, test.your_domain_name.com...etc
Production server:
www.your_domain_name.com, www1.your_domain_name.com....etc
This way you only need to one license for the domain your_domain_name.com. Note for standard license you still can only use one production server per our EULA. But we don't count development/test servers.