I have some rough sawn maple that I'll be storing for a while, I've read that as long as there's no bark on it, it's safe from new infestations of powder post beetles; is this true? Is it better to treat with borate and will this affect the finish of the wood?
I have not found any reference for the bark comment but depending where it is stored and even if you haven't had problems in the past you cannot assume you are safe from infestation. I personally have not had any issues with Powerpost beetles but the prevention thereof is fairly generic as far as infestations go. Note: there are several dozens of varieties and they have different needs as far as what woods they like and what level of moisture they feel comfortable to lay eggs.
Assuming that there is not passive infestation in the wood (more on that later) the deterrent for the beetle is that it needs to have nowhere to lay the eggs. Wood that has been sanded and finished is usually enough to keep them away. You however will be storing this wood to allow it to dry.
However an infestation could already be present and could be several years before you are even aware of it. Advice would be to kiln dry the wood or use a non-toxic borate. This would be important in your case since your wood has not been yet treated.
There are pesticide fumigation strategies that I will not suggest or condone especially if you plan on making furniture or something that will be in contact with the body.
Good Reading if you have not already found it.
I don't think it particularly matters here but in general it is good practice to mention your region as it might narrow down advice to a species.
Some pests like the Emerald Ash Borer need a living tree to infest and live, they eat the 'pulpy' layer under the bark, and in large quantities will girdle a tree and kill it. So an Ash without bark is perfectly safe.
The Powder post beetle on the other hand eats the 'dead' wood on the inside of the tree. This means that the bark has nothing to do with it. if you have an infestation you might need to kiln dry the wood to kill off any larva living inside the wood.
I think that's the best treatment you could try is to uses microwaves. The information on this site http://powderpost-beetle.com should work. It destroys worms in the depths of the wood even covered with plasterboard or chipboard.