Be ready to brace yourself for numerous snide remarks from those who'll tell you that instructors are the lowest life-form in aviation, that they're good for nothing but pottering around the circuit with amateurs and weekend warriors, and that without a solid stint in charter flying you can forget any chance of ever flying for the airlines. However it's worth noting that everyone I've ever encountered who bag instructors, invariably lack any instructing experience themselves.
I'm happy to report that all of the above is a load of BS, and whilst I do admit to having a certain amount of charter experience, I'd have to say my flying career up until recently was comprised of probably 20% charter and 80% instructing. Strangely enough it was actually my instructing background that was instrumental in landing me a job with a major airline. However I should clarify that during the four years prior, I was working in the "sharp end" of instructing - teaching multi-engine/IFR, both piston (and pressurised turbine towards the end) for various airline cadet programs. I'm sure if my instructional background had been mostly single-engine, ab-initio and other basic flying activities, it's unlikely I would have got the big break.
So in answer to the question, doing your Grade-1 is definitely worthwhile, but only if you're able to seek out the type of opportunities mentioned above. Otherwise you probably would be better off chasing those twin hours via charter instead.
In the end, I strongly believe that we all end up where we're meant to be - it's more a queston of how we get there...
P.S. As for the requirement to be Grade-1 for airline check-&-training, I recall that there has been some industry discussion regarding eventual changes to the licensing system that will require future check-&-training people to hold an instructor rating of some kind, but I think you'll find it will be a minor formality. There are lots of seasoned veterans with the airlines who didn't hold an instructor rating, but with their industry experience they would have very little trouble getting the relevant paperwork signed off. Tick-in-the-box stuff if you will.