This depends on the outcome you want to achieve.
Unannounced
An unannounced inspection requires little preparation time and lets you know immediately where the element stands with regard to standards. There is no time to cover things up.
Announced
A formal inspection requires time and resources. Formal inspections also provide an opportunity for Soldiers to prepare themselves. If you are going to ask your Soldiers to spend time preparing for an inspection, make sure you have a plan.