If one uses something to provide that income before SS begins which has a different (usually higher) risk profile, then that additional risk is something that needs to be accounted for in the decision. If one takes on more risk and seeing more return, then that is a decision distinct from and AFAICT independent from when to take SS.
I'm taking at 70 for the standard reasons, but mainly because I think the tax benefit to me of Roth conversions during the delay is pretty valuable.
When withdrawing from risky assets that aren't TIPS, there are risk concerns to consider. How they manifest themselves depends on how you withdraw from the risky assets.
When you make the decision to purchase a TIPS ladder to form the bridge, you have made the decision to take a chunk of money out of your risk portfolio to make that purchase. Whether you keep the risk portfolio with the same AA or not is up to you.
I personally would not set aside a separate chunk of risk assets solely as bridge to SS. If you decide to withdraw the amount that is projected to be your future SS benefits each year and adjust for inflation the same way, you're basically using an SWR approach with the usual risks of pre-depleting this chunk of investment or not depleting it and ending up with a large chunk remaining.
If using risk assets, I usually suggest people use TVM (time value of money) concepts from their entire risk portfolio, along with amortization to calculate the withdrawals. This involves:
1. Each year, calculate the NPV of future SS streams. The discount rate used should be the estimated expected real return of the entire risk portfolio. This will result in some dollar amount.
2. Along with your planned withdrawals from your risk portfolio, withdraw an "extra" amount that is the amount that comes from step #1
3. The total withdrawal from your portfolio is then the "planned withdrawal" + the "extra" amount.
I generally recommend using amortization to calculate the "planned withdrawal" from the risky portfolio. But even if you don't and use SWR instead, using the risk portfolio for the "extra" is going to have some risk where the risk manifests itself as extra "noise" in your year to year withdrawals before SS begins - The average magnitude of the total withdrawal, however, should be pretty close both before and after SS begins. So you shouldn't get a big step up or step down in total income, before taxes as you go through the before SS to after SS transition.
Bogleheads member siamond has an excellent 2 part series on time value of money, which also includes a sample spreadsheet to illustrate how it works. I know of at least a couple of people who have adapted the spreadsheet for their situation to calculate all of their withdrawals, both before and after SS begins.
TPAW Planner (tpawplanner.com) also uses TVM concepts as well as part of how it calculates withdrawals.
I have a bridge to SS with a TIPS ladder and I am doing Roth conversions - but only on the stock in my TIRA, leaving all of my TIPS ladders alone. I'm projecting to complete that just before RMD age of 75.
Cheers.