It does depend on the situation, but if you want to go fast without wasting effort then you should definitely follow these rules:

If you're going to want a time to take it easy, take it easy on a downhill rather than before or after. If you're going to pick a time to pedal your heart out, do it on the uphills. The reason is that it takes more total energy to go 100 meters at a high speed, than to go the same 100 meters at a slow speed.

A perfectly even speed would mean the smallest loss of energy due to air resistance, and dramatically-changing speeds (pedal hard downhill and be lazy on the uphill) are likely to lose you a lot of energy to air resistance.

Going exactly the same speed all the time usually isn't a perfect strategy because your body probably works most efficiently with fairly small changes in your effort level. Pick up your effort a little on the uphills and relax on the downhills and you may find it easier to make a fast trip overall.

Also, on multi-speed bikes it's best to gear down and spin the pedals faster when you're putting in the most effort (uphills) and gear up to make the pedals move slowly when you're not working hard (downhills).