At this point I've decided that following the entire Northern Tier route is not a good idea at that time of year (thanks very much indyfabz). Instead, my route will probably remain the same from Mass. to Buffalo, NY (even though it may include a lot of waiting out snow storms)1) I wouldn't recommend riding through Buffalo, NY. Either get south of it by the time you get to the Alden area, or cross into Ontario at the Queenston/Lewiston bridge.
2) Someone mentioned difficulty getting past Rochester. It's doable, but I don't know how well it is documented. The canal trail itself is crushed stone, so not recommended, esp. that time of year, but any time if you want to move fast, it will slow you down. From somewhere around Pittsford to the other side of Rochester proper, it is paved, and will get you through most of Rochester without much in the line of hills.
3) Weather. We (western NY) usually, but not always, have one snowstorm in April. It typically falls on clear roads/grass, and may or may not cover it to the point of being unrideable. And typically it is gone in 24-36 hours. But you can expect rain. The far south of Ontario (along the north shore of Lake Erie) is about 1-2 weeks ahead of us in terms of the arrival of spring, so you may or may get rain, but you will have no risk of snow once you get past Ft. Erie.
Your greatest risk of snow is actually closer to the Albany area, since you have more altitude there. Some kinds of (light) snow you can ride through, but the big wet flakes that stick to your sunglasses are a problem.