I was in the same situation shortly after I started beeminding about a year and a half ago. The “premium” feature I needed most was more goals and I think I probably used the “more free goals” thing at least once.
But I was hooked so quickly (by the functionality, but also, probably mostly, by this forum and the community) that I went on to get a lifetime subscription and I haven’t regretted it at all - even in times when I was beeminding less. Totally worth it.
From reading your question, @gremlinbrawler I think you should just go by feel. Use the free goals as long as you feel comfortable and when you get to a point where you know it doesn’t feel right, just sign up. In the end, if Beeminder helps you being happier/more productive/better in general you probably want it to survive and thrive and feel good with a subscription. 
[Edit: Fixed typos and missing words. I probably should beemind bedtime/sleep.]