Alright lets actually talk about this, flash on shaco. It can be good but it all depends if you are actually using it to get kills early because if you don't use it to get kills then you are most likely going to fall behind early and be useless late. Ignite is on a lower cooldown and will help you secure more early kills, because of the lower cooldown. While flash will help you kill people that escaped or get you out of bad situations. So to cut it down in stages of the game
Early fights:
Ignite takes this because if you are straight up fighting someone 1v1 and you have ignite you are going to cut the other persons healing from potions, smite, and heal so this is going to make it so they cant turn the fight as easy, while with flash you can close the distance and get more autos in or it will help you escape bad situations. Also because you have ignite and they have flash you will be able to fight more often with the summoner advantage.
Ganks:
Ignite wins this one. If you have flash on shaco and you trade flashes with a lanner then you and that lanner will have flash back up at the same time while with ignite your ignite will be up more often and because shaco does not have reliable cc the extra damage will be more useful than just always just trading flashes.
Small skirmishes:
Mostly even with a small edge to flash for its versatility. Ignite will help you kill one target better and help turn a 1v1 if and only if they other person does not have flash, Flash will help you either get on top of someone who already has flash or escape and live after you have already killed the person.
Late game/Team Fights:
Flash takes the cake here ignite does not do basically anything late game only cutes someone's healing with can be good but it isn't as useful as having a dash to escape after you kill the priority target.