Hi,
There's a mistake in the user guide, which we're going to fix. It's ok to use the "_" characters in the transaction names. There's a theoretical possibility of a problem if you create two transactions with names such as "abc@123" and "abc_123" (i.e. their names differ in just one character, and that character is "@" in 1st name and "_" in 2nd). Since Analysis will replace "@" with "_", the names will become identical and this will lead to a name conflict.
But as long as you avoid using "@" and ",", and in general avoid the case described above, there shouldn't be a problem.