Hi Stuart, I use PitchLab Pro, it works very well and I find it very accurate and stable. It has it's limitations i.e. you can only adjust a pitch value globally i.e. if you set E to be +5 cents, this will affect all E's in every octave. However for the price it is great value and works very well.
I have been using it for a couple of years with no problems. Recently I have been considering buying a Peterson tuner, I think I will probably try their app first, although a review I read in the past didn't rate it very highly and it was costing around £30. It has since come down in price, £8.50, and others have told me it works well, so will be giving it a try.
For the cost PitchLab Pro is well worth checking out and if it works for you great, if not then it's not a big loss. I started by entering sweetening values from other players i.e. Sid Hudson worked well for me, then adjusted the values by ear to suit my steel. Here's a link to Sid Hudsons sweetened values.
https://bb.steelguitarforum.com/viewtop ... ght=tuningEdit - Sorry just remembered, I don't think the Peterson app comes with the sweeteners, I believe they are an extra cost if you only purchase the app and not the hardware tuner. Will check this out.