If you are developing an extension, then please follow the naming conventions, which is UpperCamelCase for the package name as well as for property, method and event names (i.e. the first letter should be a capital letter) and lowerCamelCase for parameter names, no underscores .
Example
What does UpperCamelCase mean?
UpperCamelCase (part of CamelCase ) is a naming convention in which a name is formed of multiple words that are joined together as a single word with the first letter of each of the multiple words capitalized within the new word that forms the name.letter
What does lowerCamelCase mean?
lowerCamelCase (part of CamelCase) is a naming convention in which a name is formed of multiple words that are joined together as a single word with the first letter of each of the multiple words (except the first one) capitalized within the new word that forms the name.
All components and extensions follow the naming conventions... and these naming conventions have been setup by the MIT App Inventor team, see also the App Inventor Extensions document
EDIT: Since a while there also exist helper blocks. Helper blocks are blocks that get connected to inputs on blocks associated with a component.
Example:
The documentation of the helper blocks is here Adding Helper Blocks to a Component - Google Docs and How to Add a Dropdown Block to a Component - Google Docs
see here why naming conventions make sense
Why are naming conventions important programming?
Naming conventions help your code stay fluent within the context of your program . This is important because you want to have consistency throughout your entire program . Below you will find general rules when naming your variables. Variables tend to start with a letter.
taken from Naming Conventions in Programming | Founder at work
Feel free to use your own naming conventions in your own project, but if you like to provide an extension for others, then please follow the already defined naming conventions to make life easier for everyone.
Thank you.
Taifun
Trying to push the limits! Snippets, Tutorials and Extensions from Pura Vida Apps by 
Taifun.