Sowing seeds out of season

Ok, I know Veggie does this...and I obviously do occasionally but on other social media I'm seeing people start climbing French beans, squash and all sorts. I have got some bush Tom's going, o just had to start a couple of flowers. Really worried that I've wasted my salvia seeds as stuff grown too early and left inside can often lead to leggy weak seedlings.

I know it's only the cost of a few seeds - but I don't like wasting them if I have to buy them as the postage hikes up the costs. I've given up sowing lots more than I need as compost is precious to me at the moment too. I ran out of pots last year - I'm not going to garden centres. I'm also on a water meter - I've felt it's a waste to use all these resources on growing too many and wasting stuff on chancing my luck and then it going in the compost at the end.

I get that if you don't experiment you never learn - but climbing French beans now is madness or is it?

What early sowings have worked for you? How soon is too soon?