Short answer: no. It's not that easy to get it in EXACTLY where it came out, and it doesn't take much of a twist to be off a few degrees in initial timing.
There is definitely a difference in the way the engine will run with 0 degrees of advance versus 8 degrees of advance.
You can get it to run without using a timing light to set it, and you can even "tune it by feel," but it's still best to check it with a timing light to make sure the initial, mechanical and vacuum advances are all operating properly.
You may find that the new manifold, especially if you're going from a stock cast-iron "low performance" intake to an aluminum "performance" manifold, will be able to use more initial timing anyway.
Yup, everything he said, plus when u drop it back in, you have to make sure you arent a tooth off, it has to go back in exactly the way it came out, mark which way the rotor is pointing and make sure when you drop it back in, the rotor is pointing the same way. Yes you will stll need to time it when you drop it back in even though you arent off by a tooth. If im right, if you are off by a tooth, you cant make up for it by adjusting the timing. Good luck and dont drop anything down the whole, stuff some rags in there. Mike
Before you pull the distributor turn the motor to TDC on #1 cylinder, compresion stroke. The rotor will be lined up with the #1 terminal on the cap. I always make a mark on the dist. body and note where the vacume advance is. Reinstall the same way and it will be on the money.