I use butyl tubes in cheap Continental Ultra Sport II -- 25 on my old steel bike, 23 on the '93 Trek 5900. At 150 lbs I can run the pressure pretty low for comfort without risking pinch flats. For the 700x25, usually around 85-90 psi rear, 60-70 front; for 700x23, 90-100 rear, 70-80 front. I could go lower. But I'm mostly guesstimating -- my pump gauge and separate gauge are 10 psi apart. Who knows? I split the difference and go by feel.
As soon as I hit the first rough seams on the nearby access road training circuit I'll know whether I got the pressure right for that day. Getting the lower pressure just right adds a lot of confidence on a fast roller coaster route with some sketchy pavement on fast turns. I really want the pressure just right on that road. There are some patches of unavoidable bad pavement, especially at the bottoms of downhills leading into curves -- typical stuff found on old farm to market roads where the low spots flood regularly. There's a choice between multiple ripples running perpendicular to the direction of travel, or tire-width seams running the direction of travel with an inch high ledge, or just plain ol' fashioned potholes. I aim for the ripples. If the pressure is just right the bike floats across that stuff with nary a bobble. Get it wrong and it's terrifying at 25-35 mph.
Sometimes I'll bleed out a little air on rides when my neck is aching (old injuries). Really helps, especially on longer rides. Can't say I notice any difference in average speed, but I'm already pretty slow. A good day is averaging 16 mph over 50 miles on typical Texas rural roads. A better day is finishing it and not feeling beat up.
One reason I like the cheap Conti Ultra Sports is because they perform as expected based on the rolling resistance review site. The reviewer noted that the Ultra Sport II tires don't suddenly become sluggish at lower pressures, but maintain reasonably low rolling resistance at lower pressure without suddenly turning into glue. Big improvement over other comparably priced tires like the Vittoria Zaffiros, which are slugs at lower pressure, harsh at high pressure and just unpleasant tires. I preferred the discontinued Schwalbe One V-Guards but those were prone to nicks and cuts, although they didn't flat often. But the Conti Ultra Sport II are so good for the money I haven't bothered trying anything else for the past year. Some folks who've tried 'em aren't enthusiastic about 'em. Most folks I ride with use Gatorskins, which feel like lead filled garden hose. I haven't had enough flats with the Ultra Sport II to put up with the disadvantages of heavy duty flat protection.