"Worth it" is subjective, and depends on your uses. An SSD isn't going to improve your FPS if you're a heavy gamer. It'll help out with load times & the overall speed of your OS though.
I wouldn't build without an SSD again, but if you've never had one, you can still go on without one.
~240GB would be the minimal size I'd recommend. Even the most budget SSD is $65 minimum, and $65 is more then the difference between a 1050ti and a 3GB 1060, or between an RX460 and RX470.
I'd much rather have a 3GB 1060 + HDD vs 1050ti + SSD.
An SSD is an easy upgrade when you have the funds available, but a bit of a luxury when your budget is limited.