Where to register event handlers for a content type

Hello,

I'm developing a feature that creates site columns, content types, event receivers (class libraries) and binds all together - everything using code and not xml.  The web application should have 1 site collection and several hundreads sites. 

For example:

field1 = web.createtextfield(...); spcontenttype ct = web.createNewContenttype(...); ct.FieldLinks.Add(field1);

I have a question about binding the event receivers: Should this be done in site collection level or it should be done for every subsite?

the code is:                  

string evClass = typeof(SampleEvReceiver).FullName;string evAssembly = typeof(SampleEvReceiver).Assembly.FullName; RegisterEventHandler(currentCT, "Content Type Event Receiver", 1010, evAssembly, evClass, SPEventReceiverType.ItemUpdated); //my function.

Also, I've put the event receivers in a different project - i.e. different dll. The idea is to change the event receivers's code, copy the dll file to the gac, iisreset and have the new code running. Is this correct?

Thank you

Christos