Plain permalinks of the form /?p=14523 (or /index.php?p=14523) don’t necessarily need any .htaccess file since they don’t require mod_rewrite to rewrite the URL (unlike the the custom permalinks you mention).
However, for a URL of the form /?p=14523 (as opposed to /index.php?p=14523), where you don’t explicitly state the index document (ie. index.php) in the URL then the DirectoryIndex must be correctly set on your server. This can be achieved in .htaccess using the following directive:
DirectoryIndex index.php
Normally, this is set by default on most servers, but sometimes it is not.