Short answer:

Yes. The basic rule is “if it takes capacity of the team to get the work done, then it should be visible in the Backlog and have an estimate.”

Estimates and velocity help Teams to understanding of the capacity of the Team and reason about how much work a Team can take on. Further they provide Product Managers and Product Owners with the data needed to provide forecasts and roadmaps. In order to be used this way Team's need to ensure that anything that takes up capacity is represented in the Backlog so that:

1. As work is completed, the capacity taken by that work is also captured in the form of velocity
2. As we plan for new work, the can use the estimate (vs capacity for an Iteration) to determine how much work can be taken on
3. As we look ahead beyond the current Iteration, we can use the data generate to build roadmaps and forecasts

This means that Spikes, and everything else, should have an estimate.