There are a number of models for login/register during checkout. Some sites insist that you do one or the other. Some have an additional 'continue without doing either' option. Some signpost the 'do nothing' option as 'register later' , and provide the capability to do so.
I'd be interested to hear of practical examples as to the relative merits of the various models, and what the issues are. Has anyone implemented this type of process into an existing checkout process?