To have the “Minor Changes” checkbox enabled by default, put the following in your conf/userscript.js (create it if it does not exist):

conf/userscript.js

jQuery(function() {
    var prv = jQuery('div.preview');
    if(prv[0]) return; // preview is shown, do not modify state
    jQuery('#minoredit').prop('checked', true);
});

This enhancement is available as a part of plugin enforcesummary.

Deprecated Version (for old DokuWiki versions):

conf/userscript.js

addInitEvent(function(){
    var cbox = $('minoredit');
    if(!cbox) return;
    var prv = getElementsByClass('preview',document,'div');
    if(prv[0]) return; //preview is shown, do not modify state
    cbox.checked = true;
});