Pre 1.0, the ideal speed for each altitude was always the same because the shape of the rocket had practically no influence on its aerodynamic properties. But with 1.0, this is obsolete. Ignore any tutorials which mention anything about atmosphere and were posted before April 27, 2015.
With the new aerodynamic model, determining the optimal and maximum speed for a given altitude are quite non-trivial because it greatly depends on how aerodynamic the vessel is as a whole. Also, the orientation of the vessel now matters.
I made some experiments with a vessel consisting of just a BACC solid booster with different thrust limits, four winglets for stability and a probe-core. This is what I found out:
- You can drastically improve the resilience and performance by adding aerodynamic nose-cones or similar parts to all surfaces which face upwards. According to my experiments, the advanced 1.25m nose-cones work better than the basic ones even though they have the same drag-coefficient listed. That means the performance is better for parts which have a pointier shape.
- Avoid building too wide. The lower the cross-section when viewed from above, the lower the drag. That means you should consider to place aditional stages below instead of besides the core of the rocket.
- Avoid pertruding objects. Small, radially-mounted objects like batteries, solar arrays, round RCS tanks or science tools can create surprising amounts of drag. For better performance, hide them in the new utility bays or put the whole payload inside the new free-form fairings.
- Avoid turning the rocket while still in the atmosphere - when the trajectory isn't exactly parallel to the rocket orientation, drag increases.
When you follow these advise and go for ideal aerodynamics, the ideal thrust limit (measured in apoapsis when going straight up) is slightly below that where the nose-cones explodes due to atmospheric heating.
By the way: Since the 1.0.1 hotfix you can enable the temperature overlay and aerodymic forces overlay with F11 and F12 (where F12 is also the default hotkey for screenshots in the Steam version - you can change that in the settings of Steam itself). These can help you to diagnose aerodynamical issues of your rockets.