In your humble opinions, which I always respectfully condsider...How much bigger should your backing be than the quilted top before putting it together.

I have an top 81" wide and I cut my material last night 84" for the backing. I'm wondering now if that is too short and it will bunch up too much when I do the "quilting" process. Should I piece it at the sides and make it larger, if so how much larger?

I usually leave mine 6 inches each side but I put them on a frame so need to hook my clamps. When I was doing fmq table top I still like 3-4 inches on each side as things will smooth out as you quilt. If you only have 1.5 inches on each side it's not leaving you much.

it depends on how and who is going to quilt it...
some long arm quilters request 4"-5" all the way around---meaning if your quilt top measures 80x90 the backing would be good at 90x100
if quilting with a domestic machine you might not need as much (not attaching to the leaders which the quilt is rolled up with on a long-arm frame)
if hand quilting it-that would require something different too-
if you are sending it out to a long-armer you need to ask that person- different ones have different (requirements)
everyone is different.
i ask for an extra 5" all the way around but when there is less it can *usually* be dealt with. more is better than less though when long-arm quilting

You didn't say how you were quilting. If you are putting on a frame you need a little more. I am working on one now and I only have about 1" on each side and it is giving me problems when I quilt on the edges because my machine hits the clamps holding my quilt in place. If I was quilting for someone else I would not be doing this, they would have to supply a larger backing piece.

I usually have at least 3 to 4 inches extra on all sides. I machine quilt on a Juki.

It also must depend on how heavily you quilt. I do not quilt heavily and have never lost any excess, so 1 1/2" would be enough for me, though I try for 2".

I cut my backing 2 inches larger than the top (on all sides).

You should be OK unless you send it to a LAQ. Make sure to center your batting and top as perfectly as you can.

The denser the quilting, the larger your backing should be!!!