Questions On Serial Number Collecting

I work at a job where I see lots of bills every day. Consequently I see a lot of serial numbers that are unique. Well obviously all serial numbers are unique but I'm talking about the ones that have repeating numbers, palindromes, birth dates, and also placement of the serial number on the bill itself. I have noticed from bill to bill that there is some variation on where the numbers are placed and where the seal is over the denomination of the bill. So I guess down to the questions...

1. What kind of serial numbers do people look for in collecting? I have a few bills that only contain 2 different numbers in the serial. I have some where numbers repeat themselves like 70477047. I even have one where the serial number is a palindrome. Just the numbers though the letters are different . I also have a bunch that correspond to birth dates like 70195121 July 21st 1951, etc.

2. What is considered to be the abnormal range of placement for the serials and the seal? My gauge usually is the seal. Obviously if the serial is touching any part of the boarder than I keep it. However on the dollar for example the seal will break the word one. I have a bill where it is shifted far enough down so the date 1789 is completely outside of the word one.

3. What is considered to be abnormal within the serial itself? I have noticed that there is variations in the leveling of the serial number where one will be slightly higher that the one before it, etc. Also I have a few where the back half of a three is flat. One where half of the circle in a six is filled in. One where there is a gap between the numbers almost big enough to fit another number, but all 8 are there.

4. How important is the letters of the serial number when it comes to a palindrome bill? As I stated above the letters are different on mine. Is it still worth keeping?

These are just some questions that I've had kicking around for a while and the people who frequent this forum seem to know what they are talking about so I thought I'd ask. If anyone would like to see scans of these bills let me know and I will see if I can get them up here. Any and all help answering these questions would be very appreciated and thank you in advance for your help. Be seeing you around.