Should I wait to open all bags until level80?
Regular bags(the ones that actually look like a bag such as http://wiki.guildwars2.com/wiki/Heavy_Moldy_Bag) can just be opened whenever.
For the champ boxes it is actually better to open them with lower level characters because the lower level items(especially if they are in the level range for t4) salvage into more valuable materials. The only exception is metal ores.
The champ bags can still drop their type specific exotics on lower level characters.
Unless you want mats of a specific level range (ie. you want a chance at t3 mats instead of t5/t6), then yes, it’s best to wait to open until you’re level 80, in my opinion. The loot bags (I believe they’re actually called a ‘loot bag’) you get for personal story and such can also be saved for when you’re 80, if you want. The supply bags you get for leveling on occasion can be opened at any level because their contents won’t change based on your level.
It’s more of an opinion that a general consensus everyone does. Lower level materials are much more valuable than high level materials, because they are farmed less (because you spend most time farming at level 80). Following that logic, you should open bags as you get them. However, other people prefer getting more high level rares and exotics from the bags. These don’t drop when you’re a low level, so people who are looking to get more rares and exotics rather than lower level valuable materials opt to wait with opening their bags until they’re level 80.
I’d say if you’re interested in crafting I’d open the bags as you go because you’re going to need the materials and you’ll be drowning in the high level materials (most of them anyway) at high level, while you may hit a wall at the mid-low levels.
I give the bags from personal story on my level 10 to my level 80 to open. Usually I only get level 80 blues but I once got an exotic.
If you have a mid ranged char, you could always use that one to get those mid level mats.