While shopping for new exhaust, I have been advised to put the tallest riser under the hatch that can fit. Is this to keep from having sharp bends, to keep a wake from hitting you from behind and traveling up your exhaust, or to make the pipe as long as possible to resist reversion?

You need the ones that were designed for the boat. What boat and do can you post a pic of current risers. I agree there is some benefit to adapting new technology to older boats but that has to do with dry joint vs older wet joint manifolds. Also what year is the boat.
Chris

Thanks. Yes, I agree, risers are made to fit the boat. My point was that I am being told by the riser manufacturers that when having risers BTF, the preference is to make them as tall as the engine hatch will allow.

What brand of exhaust?? What engine??
Taller risers are used to prevent water reversion. They are really not needed on stock Merc black engines.

I'm looking at SM risers. When I called SM they said the HP 500 style tall curved risers are what they recommend as their standard. I had SM exhaust on a Donzi years ago and they were the lowest risers they made, just a very tight radius elbow, but no real curve or height. I was told that if I have the hatch room (I still need to measure), these are the recommended risers. This is on a 509 BBC about 600HP. I do not know the cam specs as I did not build the engine.

Where is your waterline on the transom in relation to the riser outlet inside the boat ?
I've seen more than one boat where the builder just dropped in a totally stock motor (normally single installations) that had risers way too short and then there was ongoing water ingestion problems . And to make it worse there wasn't any room to go up in height on the risers without re-designing the engine hatch/box .

You can't go wrong with having tall risers if you have a decent size cam and 600hp and SM is a good choice.

X 2 ^^^

Take a look at CMI Fusion offerings. They are 1 piece tail/riser/manifold and look and perform really well. They are pre set for stock Bravo exhaust locations on the boat.
At $3,800 and in stock, usually, available from Offshore Performance in Ft. Myers, FL.
And whatever brand you go with make sure they put O2 Bungs in them for you.