We use both hard cropped image sizes and flexible ones:
add_image_size( 'thumbnail', 190, 190, true );
add_image_size( 'medium', 221, 9999, false );
add_image_size( 'large', 640, 360, true );
add_image_size('huge', 950, 490, true);
add_image_size('teaser-S', 90, 90, true);
add_image_size('teaser-M', 160, 160, true);
add_image_size('teaser-XL', 460, 330, true);
add_image_size( 'largetrue', 460, 9999, false );
add_image_size( 'mr-full', 600, 9999, false );
It works perfectly fine without the “c” and $crop set to true – just try it.
The point is: Just crop every image even when it’s not necessary, it doesn’t matter – you will always get the same result.