School lets out mid June in NE so before that you should be fine without reservations. Earlier than that and you will have a lot of cool or cold spring days. I understand you like total fexibility but that can come at a cost of not being able to stay where you want.
This year it's closer to the end of June. As far as the weather goes, it's anybody's guess. April is generally on the cooler/wetter, side but not always and May and June can be warm to hot. I got married in late May and it was 95 degrees, and we had a blizzard on 5/9/77.
Starting with Memorial Day weekend through Labor Day weekend, I'd recommend reservations, especially for weekends. I'd recommend them for for Columbus Day weekend as well. Most campgrounds don't open until mid-April and close Columbus Day. The "shoulder" seasons of 5/1-6/15 or so and right after Labor Day to Columbus day are some of the best weather times. Some attractions aren't open or on a limited schedule but you get the added bonus of lighter crowds. I've camped in New Brunswick and Nova Scotia following the same schedule without issue-reserve in the on-season and don't worry in the off-season. Have only traveled a little in Quebec but had no issues. I will say they were a bit more standoffish than all of the other provinces I've been in (NS, NB, PEI, NF, ON, AB and BC) much in the same way some New Englanders are viewed. You shouldn't have any problems there.
