Rainforest now gives Landry's giftCARDS not certificates. Or at least the last one I got from MyPoints a few weeks ago was a card. It says after 12 months of non-use there will be a $1.50 per month administrative fee deducted from the balance (except where prohibited by law).
Webbies also have a dormancy fee taken out after so many months of non-use. If you get the webbies, though, you could always convert them to Disney Dollars and save those for the trip. That's what I did earlier this year, and between MyPoints webbies, test drive rewards, and survey checks, I ended up with $400 in Disney Dollars. I then used $90 of them towards my Park Hopper Pass (left just enough of a charge on my Disney Visa to get double points).
There are a few different methods to try if you want to convert webbies to Disney Dollars. I personally sprung for the $6.95 fee for the plastic card because I just didn't want to deal with any hassle from CM's at the store. Also this way I was able to put the latest $50 in webbies on my card and now I can use it to buy stuff on the trip. Others have had luck just printing out their account information from the WC site and taking that to the DS and having the CM manually enter the card number. Still others have used the WC to order a Disney GC online and then taken the GC to the DS and bought DD. You might want to check with your local DS if you plan to use one of the last two methods.
HTH!