Ok so for starters you will need a program called iBackupBot on your computer
Here is the link: http://www.icopybot.com/itunes-backup-manager.htm

So this mod will change signal bars and/or Wifi bars to a numeric value
For cell signal this value will range from -47 (the best service) to -130 (the worst service), The actual bar value for bars will vary from carrier to carrier.
For Wifi it will range from 0 (perfect connection) to -100 (weak as possible)

Step 1: Back up your phone to your computer (if you use iCloud then just turn of iCloud backup and sync)
Step 2: Open iBackupBot and find your backup (there may be a lot)
Step 3: Find the file called Library/Preferences/com.apple.springboard.plist
Step 4: open the file
Step 5: Look at the image below, for numeric signal add the SBShowGSMRSSI line with the true value and for Wifi add the SBShowRSSI line and the true value

Step 6: Save the file
Step 7: Press the little iPhone with an arrow in iBackupBot to restore your phnoe from a back up
Step 8: Enjoy!

To switch between the analog bars and the numeric values just tap them!

Here is my iPhone 4S on iOS 6:

You should help with intergrating cydia in ios 6 jailbreak...lol....

As much as I would love to it is not possible at the moment
In the mean time I hope you like this little tweak!

If you are lazy like me, here you go --- Copy and paste ready

<key>SBShowGSMRSSI</key>
<true/>
<key>SBShowRSSI</key>
<true/>

I updated my phone to iOS 6.0 and restored from a backup of 5.1.1.

It saved the settings of decibal readings for signal/wifi. You can alternatively do that as well