Whoo, tough question because these things always seem to operate in a grey area. My gut reaction is: if you're getting any kind of money, whether donation or ad revenue, the use is no longer personal.
A lot of people will say things like.. "but who will know?" Well, this is the internet and people do have trackers.
How expensive is the license? If it gives you years worth of use or permanent use I would say, pop for it. that way there is no worry. That old cliche "Better safe than sorry," has always been true.
I always copyright anything before I put it up, officially. There is then never a question as to when it was first used. I consider it a good investment as does my attorney who is a copyright attorney.
Soooo, yeah, if the license is doable for your finances, do it. Also, it would be tax deductible if you do your taxes claiming business expenses.
Of course, it's up to you, but that's my 2 cents.
Write hard, write true.
