Do you need to index your bell?
I do recommend checking runout on all bellhousings with an index plate. Yes, including the cast aluminum bell I sell. Any performance transmission setup should be within .005 runout of centerline. When I first setup these cast JZ bells I was really confident we would not have to do any indexing, we had 10 blocks we used for getting data... but after sending out 400+ of these bells we have found more inconsistencies in the blocks. We started noticing more and more customer's toyota blocks who are more out of spec than others. 

Machining Tolerances
We are within .002 on the bells, and consistent, so the inconsistency comes from the blocks... For example, i can take a few of my bells on the same block back to block and get pretty consistent numbers. But if you take one of my bells and try it on 10 different blocks the consistency does not follow. this tells me its a cast block machining tolerance issue.

What are your odds of being out of index?
While it is a smaller percentage (about 15 in 400 bells sold) that have shown an out of index condition, so I think it is better to play it safe and just check these to be sure. Out of index can cause premature failure of the input bearing as well as uneven wear and heat in the input gear. A tell-tale sign of an out of index bell can be a lot of rollover noise coming from the transmission. This is due to a side-loading condition on the input which over time can cause transmission issues. I do index plate rentals for any customer who purchased a bell or kit from me. 

TECH TIP:
To remove your stock dowels, i recommend soaking them in a penetrating oil and let them sit overnight. Use a pair of heavy duty vice grips and slowly work them back and forth. If the get tight, heat applied to the block around the dowel may be needed

Here is a quick video I did on indexing and offset dowel install: