@durhamlad - I'm interested to know your rationale for deferring your SS until you are 70? I looked at this as an option myself but having made some assumptions and done the calculations it didn't seem to be financially worth it, unless I hit the age of about 95 - maybe it's different for you as you are not impacted by WEP (which I am).
When to start SS is very much dependent on personal circumstances and I don't mind sharing ours.
Taking my SS at 70 for me is for a number of reasons but mostly as a type of insurance for my wife who is a year younger but much fitter and from a long-lived family. She only has 12 years of SS so has a much smaller benefit and has already applied for hers, plus it is subject to WEP. At 70 my SS will be over $40k/year and at that point she will get a nice bump up in her SS benefit. I think that if both spouses take it at FRA and there is no WEP involved then the lower earning spouse gets bumped up to 50% of spouse so combined income is 1.5xSS. I really don't know how much increase she will get but it should be a nice booster.
Regardless of the above, when I die her SS will be increased to match mine which is my main reason for delay to 70. (When 1 spouse dies the survivor gets the higher of the 2).
Since retiring early at age 55 and dropping down into a much lower tax bracket we both rolled our 401ks into IRAs and have been doing Roth conversions for the past 11 years. Having a SS stream of income would have slowed down those conversions for us. I completed my conversions 3 years ago and my wife will complete hers next year. My US and UK private pensions plus OAP and SS means that I will be in the 40% HMRC tax bracket well before forced distributions from the IRAs begin (RMDs), so converting to the Roth at a much lower tax rate is very much advantageous to us.