Getting the endorsement of a newspaper is never a bad thing. While the value of such an endorsement is diminished these days, the Des Moines Register's blessing of the campaigns of Marco Rubio and Hillary Clinton won't be a net-negative, it's safe to assume -- both because endorsements in general are rarely net-negatives and because the endorsement of the Des Moines Register has, in recent years, been followed by jumps in the polls -- though modest ones.
The Register began endorsing for the Iowa caucuses in 1988 and took 1992 off, since Iowan Tom Harkin was a shoo-in to win the state. That means it has endorsed nine non-incumbent candidates for the presidency. Every Republican it has endorsed has won either the caucus or the nomination; no Democrat it has endorsed has done either.
FiveThirtyEight has looked at the extent to which the Register's endorsement seems to help candidates, finding the biggest poll increase after it was granted came to John Edwards in 2004. Edwards still lost the caucus -- and the nomination and the vice presidency and, ultimately, any goodwill from the American people -- albeit for unrelated reasons.
In 2008 and 2012, the paper's endorsements came before jumps in the Real Clear Politics polling average for each endorsed candidate, though it's not clear if there's a cause-and-effect relationship.
In 2008, Clinton was trailing Barack Obama when the paper endorsed her. Within the next seven days, she had tied it up -- and then took the lead going into the final week. In the final tally, though, she came in third, as Obama and Edwards surged past.
Change from average on day of endorsement to final result: +4.1
When the Register endorsed John McCain that same year, he was trailing Mitt Romney and Mike Huckabee badly. After the paper's endorsement, his poll numbers jumped up -- but it was nowhere near the sort of boost he would need to actually win.
Change from average on day of endorsement to final result: +7.3
In 2012, the paper backed Mitt Romney. Romney had slipped behind Ron Paul by the time of the endorsement, but quickly saw a big jump -- likely not due to the endorsement, given how soon afterward it followed. He continued to improve as the caucus neared, and came out on top in initial tallies. After all of the counting was done, however, he was edged out by Rick Santorum.
Change from average on day of endorsement to final result: +7.5
The 2012 example is interesting particularly because the endorsement came as the race in the state was close, and the endorsed candidate ended up essentially-but-not-quite-tied for first.
It's hard to extrapolate from these three examples (or even the nine in total) to make assumptions about how the endorsements will affect Clinton and Rubio. It's not a bad thing, as we said! It's just not clear how much of a good thing it is, either.