The master bath has teak, but as you noted, it has a pedestal sink top on it. The countertop itself is still in pristine condition after...honestly, I've lost count of the years.

I recommend a gloss polyurethane for the first few coats for durability, then changing to a satin poly for the for the finish coats to take the sheen down a bit. Four coats is what I used.

My kitchen has a stainless sink undermounted on a teak countertop. That counter, like the bathroom tops, is edge-glued (actually epoxy) teak, although the kitchen tops are 8/4 (2" thick) planks where the bathroom countertops are 6/4 and the windowsill 4/4 (1" thick).

With the kitchen being heavily used and prone to dents and dings, I didn't want a film finish like poly on it. So the kitchen countertops, both the edge-glued plank main countertops as well as the end-grain "butcher block" top, all have a basic wipe-on mineral oil "finish" on them that I renew maybe every 6 months or so.

The kitchen gets heavily used and I probably let the sink area go longer than I should in term of renewing the mineral oil there. But there is zero damage to the wood around the sink. No end-grain checking no nothing. The teak still looks great.

Because I prefer a non-film finish in the kitchen, especially around an undermount sink, teak is my recommendation due to it's durability.

For a master bathroom where the countertop will see gentler use (no dropped cooking pans or cans or jars to dent the countertop and damage the film finish), I think any decent wood is fine with an undermount sink as long as a protective film finish like polyurethane is used. The poly will protect the wood from most any and all lotions and potions, as well as from water damage.

Poly all sides of the countertop before you install it. If you only do the top and edges and not the bottom, you could get uneven seasonal movement of the wood and cupping of the top as a whole, perhaps even some cracks.

A few years ago I got a good deal on some south american mahogany, I used some to make a front entry door for the house and some to make a large "L"-shaped mahogany countertop for my office. I finished the countertop with polyurethane. I'd use that in a bathroom. But most other woods are fine too as long as they are protected with a film finish like polyurethane.

FWIW, all of my tops (the teak and the mahogany) are planks 6" to 8" wide that are edge-glued together with biscuits and epoxy. Titebond instead of epoxy work well too. If you use a less stable flatsawn wood, then ripping the planks into thinner strips and flipping every other piece to alternate the grain, then reglueing to get that faux "butcher block" look would give a more stable top than the more simple edge-glued wide-plank top.

If you shop for lumber and find "vertical grain" wood, in general because of how it is milled, it will be more stable than "flat sawn" planks.

In the above, if you look at the wood grain on the end of a piece of wood, if the grain lines are curved like "smiles" or "frowns" then that is generally flatsawn, or as they depict it in the drawing, "flatgrain" lumber. Flatsawn might stay flat in a countertop, but it is more prone to cupping across the width of the board. But you can take a wide piece of flatsawn lumber, cut it into, for example, 2" wide pieces, and flip every other piece over so the smiles and frowns alternate. Then glue it back together. That'll potentially give you a more stable top.

If the grain is a bunch of vertical lines ("vertical grain" or VG), or close to vertical, then you have a piece of wood that is less prone to cupping and it could be used as a wide plank.

All of that is probably too much information. And to be honest, one grain pattern over the other isn't the only ingredient in terms of being a recipe for success or failure. I don't mean to get too technical, but if you have the choice of selecting your wood based on grain pattern, why not chose the better piece? To be forewarned...why not?

Still, despite grain, the biggest thing you can do to stabilize the top as a while is to finish the top, sides, and bottom of the countertop with equal coats of your chosen finish.

Switching gears...In my kitchen I do have a true teak end-grain butcherblock island end cap, about 4' square and 5" thick, that I assembled with epoxy. It still looks great, but there is one minor grain check on one of the 5" sides.

I my area where wood can be pricey, teak has gone through the roof in recent years, $30-$40 a board-foot. South American Mahogany (NOT Phillipine!) is around $8. But again, any decent wood can work for your purposes.

I guess that's the v-e-r-y l-o-n-g answer? lol

The short answer? Yes, wood in a master bath, if fully polyurethaned, could work with an undermount sink!

And finally...maple, though a common wood for countertops, does move more than most other woods.

Best of luck!

(now I'll step away from the keyboard...honest!)

Thanks very much for such a detailed reply, Mongo.