When to use Sanitary/Food Grade Piping

I dont think there is such a spec that will tell you for definite that food grade piping is needed.
I work for a consultancy company that does a lot of work for food industry clients and typically the client would specify what pipework they want installing.
If I understand correctly - you're company produces chemicals and one of your customers uses your products as an ingredient. I think that the FDA would require material traceability from the food manufacturer and they would request this of you - therefore they would be telling you when food grde piping is required.

My determination would be how often do you plan on taking your system down to sanitize?

If it is frequent (once a week?), I'd go with the sanitary fittings since they are easy to disconnect and reconnect.

If your process is continuous with little chance of contamination (due to acidic nature for instance), I'd go with regular piping (flanged and welded).

On welds though, make sure you specify to grind and/or polish them on interior surfaces so that you don't get pockets of material that can hang out for a while and ferment or become contaminated.

Talk to your tank manufacturer about what grades they offer and the costs associated with each. They will generally have built something previous that is similar to your needs.