No. Pressure is only a factor when the yeast are active (ie, you don't want a lot of pressure in the carboy).
quote from the wyeast faq:
If you have activated a package but can’t use it right away, simply allow the package to swell and then refrigerate the package. The yeast will survive for extended periods if refrigerated. Before using, take the package out of the refrigerator and allow it to come up to room temperature before using. It is best to use the yeast as soon as possible.
You don't need new yeast. If you are worried, make a starter. What is the date of manufacture? I've used an 8 month old pack (with a starter of course).