Ideally I would always like to be involved once the requirement has been identified and before work is undertaken to specify the goods or services that will be sourced. Unfortunately we often have to work with specifications which have been drawn up by users who, whilst they understand their own needs, they are not aware of what is currently available in the market.
Suppliers may have in fact developed better or more cost-effective ways of meeting our needs. By not ensuring the user has investigated the market offerings before the specification is drawn up, we may end up either paying too much, sourcing a less effective solution or have to draw up a new, more approporiate specification which adds time to the procurement process.