Generating passwords from a master password using rules
The reason reusing passwords is a bad idea is that the security of all your accounts relies on the security of the weakest account which uses the same password. If you sign up for PNC Bank and [favorite show here] Fan Club using the same password, one of them being compromised will compromise them both.  

That being said, under the assumption that one of these services will be compromised eventually, the security of them relies on the security of your scheme, rather than the master password itself. If all you're doing is changing the first letter, then it is trivial to guess it given a leaked password.  

A better method could be using some variant of HMAC to combine your master password with the website's domain name.

HMAC_MD5("foobarone", "facebook.com") = "b5bda098afd19b1f7992abfa9ab7d710"


You could then use "b5bda098afd19b1f7992abfa9ab7d710" as your password for facebook. For convienience, you could truncate it to "b5bda098afd19b1f" and still have a relatively strong (64 bit) password.  

Using LastPass (or some other manager for that matter) is better than using a homemade system, though.