Using 'abbreviations' in a informal letter

 I realized that Grammarly software doesn't correct 'abbreviation' for informal writing.  Example, if I use a word 'we're' your program should suggest for me to change it to 'we are,' because in formal writing the rules apply that you should never abbreviate a word in a informal letter, business plan, or proposals (casual letter is OK).  Let me know what you think.