No reserve auctions will attract the most bidders, but also have the highest risk of taking a loss if your domain sells for less than what you have invested in it. You can offset this by setting the starting bid at a certain price, which in way is setting a reserve price. This can equally make bidders pass and move on to the next auction.
A lot of it also depends on the liquidity of your domains as well. A good 4 letter .com is going to favor well at most auctions so the risk is lower if you don't set a reserve, whereas a random two word domain may see little activity and thus have a high risk of selling low. You need to evaluate the domain, current marketplace, and platform to determine what auction strategy to go with. It may change if you switch platforms from say Namejet to Godaddy to eBay. A lot of it is just luck as well.