If I make a successful run on the HQ or R&D, do I have to declare I'm going to use a Mole before watching the cards I would access, or can I access them, and then decide if I want to access more (HQ or R&D Moles) or trash them (Mercenary Subcontract)?
There is no official rulings, because an official (Wizards of the Coast) hasn't yet ruled.
The last official rules errata was v1.7 from March, 2000.
The next official rules errata v1.8 has been stalled, you can read the discussion thread here. There is errata concerning the Moles cards, but I do not believe that WotC has provided an official ruling regarding the matter within their mailing list.
It is possible that WotC hasn't ruled on the matter, but netrunner online (not official), may have come to a decision. This can be observed on their rules page.
I don't know of a specific official ruling on this, but I believe you can access the cards you would normally access, and then decide if you want to use HQ or R&D Mole to view more.
This is based on the fact the runner does have the ability to use cards (such as Mercenary Subcontract) that affect the cards currently being accessed, and that the Runner can intermingle card draws with accessing upgrades
("If you are accessing multiple cards from HQ, you can choose the order in which you take picks, and intermingle them with accessing upgrades as you wish. (Sparky, Netrunner-L, 11/18/96)", from errata), which seems to imply an ability to use abilities while accessing , even though it's not listed as one of the times the Corp can rez cards ("Either player can use special effects during the same times the Corp can rez cards. However,if the effect requires a cost of one or more actions, it can only be used during that player's turn. (Netrunner FAQ v1.0, 5/23/96)", also from the errata; "We can only rez other cards at the following times:
Edit: This appears to be a subject of debate on boardgamegeek. I agree that it would simplify several things to errata the card to "Use this ability only when you are about to access cards from R&D.", but as written it seems to allow use after accessing cards. The main point against it is this bit from the errata: "The Runner chooses the order in which he or she will access cards, including any draws or face-down cards as well. That's the order in which they affect the Runner. If the Runner survives accessing all the cards there, the Runner can then score agenda and pay to trash nodes and upgrades. (Sparky, Netrunner-L, 6/12/96) The effect of each accessed card must be dealt with separately and immediately, before accessing the next card. (Sparky, Netrunner-L, 11/18/96)". This seems to suggest that while the Runner can choose the order, they may not be able to initiate further accesses after accessing some cards. I would probably suggest making a house errata as suggested above, in order to simplify things.