Typically, on
DDP (
Disney Dining Plan, aka the "Regular" Plan), spending 2 TS credits for a meal is not worthwhile for adults, but can be for children. However, if you're on the free dining plan promotion, there is a bit more leniency (since the actual cost per credit would take into account the money saved over a room discount, and thus be likely lower than paying full price). Also, it's better to use credits at lower value than not use them at all.
---Start Nerdy Math Stuff---
Cost = the price you pay for the credit
Value = the price of the item you receive for the credit
Through some tricky math that Faldred (not sure if he still hangs around here) came up with, we were able to discern the cost of the various credits of the various plans. We did this by using a standard $3 value for the snacks (which is about average and works out well).
Through these formulas (the actual formulas are somewhere around the boards, unfortunately, I lost my bookmark sometime over the past 6 months that I've not been DISing

), we discovered that the cost of an Adult TS Credit, for 2012, is roughly $36. The cost of a Child TS Credit for 2012 is roughly $10.
This means, to make sure that a particular meal would come out ahead, you'd want to make sure that you spent at least $36/$10 per credit (Adult/Child). Since the HDDR is less than $62 (erm, I think?) and most signature meals would be less than that (especially without an app), spending 2TS on a single meal (again, for an adult), may not be worthwhile.
---End Nerdy Math Stuff---
However, this should be taken with a grain of salt, as undervalued credits are better than unspent credits (or even adding a meal or two that you didn't intend to do in the first place). You can also underspend on a couple credits, and make the difference up elsewhere. It's just one method to help you pick and choose the best use of your credits for your own situation.