So there, as usual, the main thing is a balance between the budget the client has, the size of the room and the theme.
These are the limitation. If you have an unlimited budget - great! Get a huge room, let the food, flowers, balloons decorations, lighting and even the firework be all color coordinated. However, it does not happen very often... Never in my experience (we are in business since 1990). Interestingly enough, I have met an event planner that has that situation. In 2001 he was doing events with budgets 1 to 5 million dollars.
Back to reality. Most events have a little less than that. So, as always, the best results come from the best distribution of the resources. The main idea here is what is noticeable and what is not. The main parameters here are size of the decorational object, how well it is lit and even if it moves or not and so on. If the object is relatively (very important!) small, it better be a very well lit diamond, otherwise nobody will notice it. Money is lost. Why? If, for example, you have an event outside, you see trees, clouds, may be tent. Same is true if you have a hall for 750 people. Small objects will not be noticed. So, if the budget is limited, big objects make more since. (by the way, the most economical big objects made out of balloons, any other material is more expensive) and they have to be themed.
Then next step down is probably relatively large decorations around the room, something like balloon columns of any sort. If your theme is Music, the top of the column can be a mylar balloon in the shape of note, a guitar or a saxophone. Then, probably, the centerpieces. The usual mistake of a lot of my customers, they start from it and want to theme it to death. It is a good idea if the ceiling and the budget for decorations are high. Then we can make tall centerpieces with the element of the theme and make it fly high. Which will take noticeable part of the budget. If the ceiling is low - even with enough money, you cannot do a top heavy centerpiece - it will not look good!
Yes, we can do everything small and cute but... As soon as you get of the main truck and want to get, say, even the same saxophone 2" high, you will pay more, as always if you need something special. So, it is small and cost more. Solution? The smaller (and the less noticeable) an object to be decorated is, the less theme should be pushed in there. Color will be enough.
Now, it is time to say a few words about exceptions to everything said above.
First, all of the above is more or less time tested theory and it works to make a party. But you do not have "a party"! You have yours! And it has, no, it must to please you, not the theory! So some people want it according to the rules. But someone wants it ugly and expensive! Against all rules. Wrong (from general prospective!) choice of colors and shapes, too many people in the room, wrong music and theme is everywhere including toilet paper. They must be able to get it. My customers do.
Second. There is an exception to the "big object" rule. It is small object. A lot of it. Let me give you an example. My own friends had a wedding in a very large stretched room with low ceiling. The room was good, because one wall was all windows facing the ocean, less than 50 yards away. But big object option was ruled out. So we came up with a small cluster of golden and maroon balloons, with golden ribbon and covered all visual space with it. All ceiling and part of the walls. On one hand, you "do not see it" when you walk in the room, it does not strike you, like it is in the case with balloon dinosaurs. But it always in your eyes, no matter where you look. All the same, very consistent, even ribbons move all together. Very effective. Very expensive. But - breaks the rule!