gcpursuit,
Whereismygc is updated - whenever there is important data available that would impact the forecast. Sometimes it could happen twice a month - sometimes once in two months. I hope that answers your question.
As per whether to use monthly or annual (or even free service) on wherismygc.com; if you are looking only for directional answers - qesehmk.org blog has pretty good directional answers from various people/gurus and so you don't need the paid subscription to whereismygc.com
The paid service at whereismygc.com will be very useful if you are looking for a personalized forecast and/or you want to create your own forecast by adjusting some assumptions. Hope this clarifies ..