eta keys are almost always safe (<M-A>, <M-B>, etc.). Check for mappings from plugins, etc. before using. When in doubt, use the help. For example, to see if CTRL-J is taken (it is, but there are several commands to do the same thing) type :help CTRL-J and then press CTRL-D instead of ENTER to list all results. Then view each result to see what they do and if there are any alternate ways to do the same thing before mapping. If the mapping is for one mode only, you can prepend the mode. For example, for CTRL-J in insert mode, :help i_CTRL-J