no. not unless you do it toward the end.
for example, i buy my one year card around september 2004. It keeps my service active untill sept 2005. it also gives me 150 talk minutes. (sometimes, like right now, they offer an additional 100 minutes for free). Then around march, when i start running low on minutes, i'll pick up a 50 card (gives me 200 more minutes, but only 2 months of activation). the additional 2 months of activation runs concurrent with my 1 year card, so it dosen't help in that respect. but i typically use the cell phone less than 30 minutes a month, and it ends up costing me less than $12 a month for the security of having a cell phone.
for some, like me, it works out good. for others who need and use a cell phone a few hours a day or more, it's expensive.