The beginning of May is a time when the greenhouse is full to bursting point all the plants are fighting for space. But it is still too early to put them out in the garden due to the cold nights and wind that will knock them back and at worst kill them. If you heat your greenhouse it's best to harden off your plants first it is too much of a shock for them move them into a cold frame several weeks before you want to plant them out. But if you haven't got a cold frame like me i just reduce the temperature slowly until it gets turned off completely this way all the plants get slowly hardened off.
One of the problems i find when hardening off is you get a lot of condensation and damp either in the cold frame or greenhouse this can lead to a lot of fungus problems so you need to keep them well ventilated. It also helps if the plants are well spaced out so that air can get between but if you do get a out break of fungus and rots just give every thing a spray with a fungicide.