Do I have to install the tub spout portion of a tub/shower trim package?

You are not telling us what do you exactly plan on doing, what you want to accomplish and what type/brand of faucet you have...

But in the interim...

No, you don't have to install tub spout with tub/shower set up. In fact, many manufacturers supply tub/shower bodies even they intent to sell it as shower only or tub only faucet.

If you want to use it as shower only faucet then you plug the bottom (spout) outlet at the rough body and use only upper outlet (shower). If you want to use it as Tub faucet only than plug the upper outlet (shower).

This works well with single handle faucets + diverter spouts. You can do the same with the older model 3-handle faucet - but the only drawback will be the non-functioning handle in the middle.

Important Note: If you have older Grohe tub/shower faucet with diverter spout than your shower will be routed from the bottom (spout) outlet - not from the top. Here you just won't use the required "Brass Ell" fitting.

Hope that will help... Good luck with your project... Milo