Once the water temp rises above 60 degrees the algae can thrive with a FC of zero.
I opened, my water was 52 degrees and clear. The forecast for the weekend was upper 80's so I thought I'll turn on my heater and we'll have our first April swim. Took 24 continuous hours and the water temp reached 84. Unfortunately the outside air never did, and it rained all day, and then late afternoon the temp dropped to upper 40's. Needless to say we didn't swim.

and I wasted all that gas for nothing....

as the air temps are forecast to be in the 60's all week. That's what I get!
So is up to you. If you do open, and the temp outside drops back down, you can keep your pump run time lower until the temps rise again. So if you would normally have say a 12 hour run time in the summer, you could try 6-8 and see how you do, and even split it up 4 during the day, 4 at night or whatever. The warmer the air/water the more filtration time you'll need, keep in mind if you are in an area with lots of trees you may see more crud in the baskets than usual, and skimmer socks would be good since it is pollen season.
Keeping a solar cover on will cut down on evaporation and uv consumption of the FC.
I opened early because I didn't want to deal with green water. I didn't have to shock, as I still had a 3.5 residual from last fall.
If you do open and water is clear, and circulated, if your FC is still 0 or low, you could just bump it up abit to your normal range, do an overnight FC test and see if it holds. If it holds you don't need to shock it. If it drops overnight by more than 1ppm, then you should shock it.