Nationwide strikes have become more common in the last couple of years but they seldom last that long. Last week's strike was the last one in a series but with 8 days it was the longest in many years.
Also, I have travelled during strikes. It's a bit more difficult because you have to be prepared for delays, crowded trains, and impromptu announcements in German but some trains are still running.
Furthermore, if your train is cancelled because of a strike, you can get your money back even with otherwise non-refundable discount fares. So waiting to buy later would be more expensive no matter what and it always make sense to buy in advance.
Incidentally, most alternatives (long-distance bus, car hire) have a similar price structure so it's best to make your decision as soon as possible. If you are willing to choose one of them right now, then go for it but if you are still considering the train, don't wait for the last minute to buy a ticket.