I think I would wait to be further along to tell them, because you're right- no sense in telling them, having them get upset, then a miscarriage happens and everybody feels horrible.
Take the time now to make a budget (if you don't already have one), being realistic about the $ you make, the expenses you have, and do some research about the costs you will have raising the baby. Figure out work schedules- will you need day care? How will you pay for that? Consider cloth diapering instead of disposables, breast feeding instead of formula feeding. If you can't make the finances add up, decide how you will increase your income, and start working on that. Will one of you need a second job? Start looking now.
In addition to that, consider your future goals. Do you or your fiance want to go to college? Will that be possible with the baby? When and how?
If you can answer these questions, then when you tell his parents you will already have a plan and you will sound more mature and ready for this responsibility.
And in the end, you both want this child and that is what matters. Even if his parents aren't happy at first- chances are good that they will fall in love with their grandchild. As long as you both take the responsibility of being parents seriously and think of your child first and foremost, it will all work out. Good luck!