I wonder whether it is worthwhile to remove the stock periodically to clean the underside of the barrel, action, etc. Do you think it is worth the trouble? My #1 rifle is a Sako 75 hunter (blued/wood). I have never removed the stock, but I am contemplating whether I should to do some cleaning in there...
What do you guys practice?

Not a bad idea periodically, especially if it's been soaked. While you have it off, degrease it and use some good car wax to protect the metal next time you get caught in a rain. It's not much trouble really.

If I end up hunting or shooting in rain or snow, I will remove the action and parts from the stock for cleaning and a light coating of protectant. I don't do it on a regular basis, wouldn't hurt though.

Back when I had nothing but constantly Rusting Blue and Termite Food rifles, I took them out of the stock quite often. When used in the rain, every night. Some required resighting and needless to say they didn't get used much in inclimate weather.

Had some of them Parkerized and it is an absolutely excellent finish for a rifle which actually gets to hunt in all kinds of weather. During the Summer heat you remove them from the stock, coat the entire Action with a grease and set them in the sun, which pushes it into the Phosphate. And putting Synthetic Stocks on them seriously reduced the need to re-sight every time a stock was off. The ones which still needed resighting either got fixed, traded or sold.

Now I have a lot of Stainless and Synthetic rifles which come out of the stocks when they get soaked. They get wiped down and then dried really well with a Blow Dryer. And they go through the Summer Greasing too.

If I know I'm going to be in a long distance situation, I still feel better about re-verifying the Point of Impact has not shifted once a stock has been removed and replaced.
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Oh yes, I have a regular routine where I put the Stocks back on the same exact way every time and I have "Witness Marks" on the Stock Screws so they retun to the same position they were previously in.

I put the Action back in the Stock and put the Stock Screws in loosely. Then I place the Butt Pad on the Floor with the barrel straight up and tighten the Screw closest to the muzzel so it is good and tight without being Gorilla Gripped(to the Witness mark). Then I tighten the screw closest to the Butt Pad so it is snug, but not as tight as the Front Screw(also to the Witness mark) and I'm done.

If I had a rifle with a 3rd screw in between those, I'd ease it in just enough that it wouldn't fall out, but it would not be considered tight at all.

I only shoot targets and never in inclement weather. So, I have never separated the action from the stock. Reasons?

1. "If it's working why fix it"
2. Not game to. One of the rifles I own was glass bedded at the start by a competent gunsmith. I reckon with my luck, if I took it apart it would never shoot as good as it does. To back that up at the last range visit (2 weeks past) 243WIN using 55kg Nosler in 3 shot test loads at 100 yds group size was .266".

All of my blued steel rifles & shotguns get degreased with acetone and or alcohol then coated with Trewax http://www.trewax.com .Also use Old English Polish to cover any scratches and a final coat of Trewax on the walnut stocks.A very learned master gunsmith taught all of his customers about the care and maintanence of their custom weapons before they were removed from his shop and everyone I know still uses Trewax on their weapons. Scant amounts of Oil & Grease are only to be used in the internal surfaces of weapons.

If you have Jim Carmichel's "The Book of the Rifle", you may want to reread his section on taking down a wood stocked rifle. He recommends it be done BEFORE you shoot it the first time, especially with a factory rifle. This allows you to see if there are any hidden defects, lets you wax the inside of the stock as well as the barrel, and forces you to tighten the action screws as they should before you fire it. It's all good advice.

When you replace the stock you may have to shoot a few rounds for the metal to seat in the wood properly ! I have not found this true with a properly glass bedded stock.

I have a fluted barrel and dirt and stuff get into the flute grooves so i take the stock off, no change in POI.