The answer is: only the GMs truly know.
I hear a bunch of media people saying, "Oh they're going to get a better deal during the summer" because their answer to everything is "wait and develop", yet I can name a whole bunch of summer deals that didn't pan out for the teams this year, LA and CBJ getting the Philly players within the hour last summer both turned out terrible for them. Philly took LA for a ride and stripped them of two upcoming roster players for a guy whose numbers had been on the decline. CBJ traded away too much to get a player with somewhat of an injury history and a very questionable chemistry on his team.
And they don't get trials either. When you get a player in the summer, chances are that unless he's injured, he's playing at least 60 games for your team, lest you risk dealing him for someone who might not pan out. That's asking for that dreadful slow start to the season that might suck the wind away from your sails. And that's exactly what happened to Columbus, Montreal (losing key defense), and to some extent, Los Angeles. I expected it to happen to Philadelphia, but the acquisition of Jagr and the breakout year of Claude Giroux as well as the emergence of Simmonds and Read saved them from what might have been a year spent in the margins.
(In the margins = think Toronto, Calgary and Florida, who are always on the fringes of making the playoffs but usually miss them)
And then you have the people who say they should make the deal at the deadline. They're the Healy types that want to see everything happen here and now, who are impatient and want stories and conflict to erupt all over the league so that they're more interested in driving to work. (Who wouldn't love a job in hockey, though?)
But it's not like we see the offers on the table for these players; this isn't the Price is Right where the GMs have "next item up for bid" and they're making guesses with people trying to shout out answers from the crowd. We hear "rumoured offers" at the deadline and "rumoured offers" in the summer, and unless we hear confirmed and rejected offers, then what we hear is "nothing".
So to objectively view where and when, say, Rick Nash might have been dealt to get the best return, we'd need to see examples of these offers. Past GMs might have input and say "well the summer is better" but if the summer is always better, why are some people trading at the deadline? The media contends against itself, some wanting trades to break so that they have more news, and some saying the summer is better simply out of wanting to sound wise and patient.
Ultimately every situation is unique in the hockey world and there isn't always one right answer. For all we know, teams interested in Nash at the deadline might offer less during the summer because they have many more trade partners to get what they're really looking for instead of desperately trying to bring some star power in a team in an attempt to make the playoffs. Then teams interested in Nash during the summer might offer a bigger name and take their draft picks off the table so that CBJ can get a team that can be competitive in the fall and not in two or three years time, but in the end, get less.
We don't know, so don't pretend anyone knows what they're talking about when they're saying Summer or Deadline. The only people who truly know are the thirty guys who actually see the offers and their staffs that discuss the offers and offer limited insight to the public on the deals offered.