I made a design and a specified the wrong package for a cap. I specified an 0402 10UF tantalum cap. the issue is that I can only get them up to 6.3V. Do you think this will be alright on a 5V input power line?

 

I think it should be fine.

 

I second that.
As long as you can trust the 5 volt supply to hold steady, this will work properly for a very long time.

 

For a protytype or something to work for a short duration it should be "just OK." You can get away with it.

I would never select a cap rated for less then twice the typical voltage it is nominally seeing. That's based on the mean time before failure (MTBF) calculation from MIL-HDBK-217.

Those caps are damn expensive, though cheaper then a PCB re-spin. You might be able to slip in a larger part such as an 0603 to the samefootprint, and those DO come with a 10V rating (but still two bucks).

 

Fairly different point of view here. I did some research on aluminum electrolytics and found recommendations that one would not use more than twice the actual voltage because there is no benefit in MTBF for that on aluminum capacitors.

Then I look at the fact that I only learned that in the last few years, but I have projects I built 35 years ago that are still running, with the capacitors rated at barely over the necessary voltage, for instance, a 50 volt cap on a 39 volt supply. Just lucky?

So, whether you are designing a production run or a hobby project decides your point of view.

 

Then I look at the fact that I only learned that in the last few years, but I have projects I built 35 years ago that are still running, with the capacitors rated at barely over the necessary voltage, for instance, a 50 volt cap on a 39 volt supply. Just lucky?

No not lucky. Some literature assumes more is better. Capacitors, in particular, is an exception. IMO, 20-25% margin is perfect.