MAC addresses are used within the scope of the "layer 2" (ethernet) network. On the other side of a Router (which does "layer 3" forwarding), the packet will use the different layer 2 (MAC or other equivalent) address for that medium, and so on between each layer 3 hop.
If some remote site is seeing your MAC address, it would have to acquire it through some odd means (e.g. in a packet's contents, being reported by spyware). I suppose they could correlate it with other information, but it doesn't contain a lot of useful information. Translating it to the manufacturer is easy enough through online databases, but identifying that manufacturer's assignment process during manufacturing isn't always so easy. So all they could use it for might be some sort of machine ID. There are certainly more popular ways of doing that, and your browser probably reports more information about your machine every time it connects (e.g. browser type and version, operating system). Unless you want to spoof a new MAC address on a frequent basis (meaning that you suspect some spyware might be reporting it!), you won't accomplish much.
Then, any spyware which was strange enough to report a MAC address would be expected to watch (and report) any changes to that MAC address, correlated with some sort of cookie.
Short answer: it's unlikely that anyone beyond your ISP can see your MAC address, and spoofing it is not likely to bring any benefit.