If you find yourself needing to manipulate node fields during hook_node_insert or hook_node_update, the best way is to use field_attach_update. Especially, if you need access the node id.
If you're looking to preprocess a field in Drupal 7, you can use hook_preprocess_HOOK. The first "hook" is the name of your module. The second "HOOK" is the theme hook you want to preprocess (e.g. theme_node, theme_block, theme_link, theme_field). You can fine an exhustive list of the Drupal theme hooks by searching the Drupal API.
Here is a simple example of how to use it with fields.