I personally think it's a personal preference. I posted a post about this in the nursery forum. Most people who responded aren't buying a new mattress. My parents bought us a $90 Kolcraft mattress for DD and we used it. It laid in the bedroom without sheets for a while so it didn't stink by the time DD used it a couple of months later. Now our mattress was stored in a dry area in our garage and it's still in good shape so we plan on using it for #2. If it was soft at any spot then I would most likely get a new mattress. I will buy a new mattress pad for it though.
Annie
WOHM to two wonderful little girls born in April
DD E, 17
DD L, 13,
baby 2, 4-2009 (our Tri-18 baby)