If, like me, you want to reserve the first x number of node IDs for categories, here's how to stop the "Main Forum", which has a node ID of 2, from being created during installation.
This might seem an odd thing to do at first, but categories are the only node type for which you can't (currently?) define a URL Portion/Stub and as such, the node id is always appended to the URL, for example: http://xenforo.com/community/#official-forums.1
Open the following file: library\XenForo\Install\Data\MySql.php
Find the following code around line 1780:
Replace it with this:
Install the software as normal and after installation, there will now only be 1 node which is the "Main Category", as follows:

Now you can create as many nodes as you want as categories (keeping some for later use as necessary) and the numbering will be sequential.
This might seem an odd thing to do at first, but categories are the only node type for which you can't (currently?) define a URL Portion/Stub and as such, the node id is always appended to the URL, for example: http://xenforo.com/community/#official-forums.1
Open the following file: library\XenForo\Install\Data\MySql.php
Find the following code around line 1780:
PHP:
$data[] = "
INSERT INTO xf_node
(node_id, title, description, node_type_id, parent_node_id, display_order, lft, rgt, depth)
VALUES
(1, 'Main Category', '', 'Category', 0, 1, 1, 4, 0),
(2, 'Main Forum', '', 'Forum', 1, 1, 2, 3, 1)
";
$data[] = "
INSERT INTO xf_forum
(node_id, discussion_count, message_count, last_post_id, last_post_date, last_post_user_id, last_post_username)
VALUES
(2, 0, 0, 0, 0, 0, '')
";
Replace it with this:
PHP:
$data[] = "
INSERT INTO xf_node
(node_id, title, description, node_type_id, parent_node_id, display_order, lft, rgt, depth)
VALUES
(1, 'Main Category', '', 'Category', 0, 1, 1, 2, 0)
";
Install the software as normal and after installation, there will now only be 1 node which is the "Main Category", as follows:

Now you can create as many nodes as you want as categories (keeping some for later use as necessary) and the numbering will be sequential.