It all depends.
For me, I normally buy according to what project I am working on. If I can find a matching set/pad/pack of papers and embellishments to go with my project then I go that route. When selecting a paper pack, I make sure I like at least 90% of the paper included to make sure there wont be any wasted/left unused. The ones I don't like for scrapbook pages, I use on cards and that typically works because cards use much less of a paper which makes some of the less desirable patterns easier to deal with.
HL & M's always have sales on paper and things on alternating weeks it seems and both offer coupons! Typically I use the coupons on tools that can be used over and over. I don't see the sense in using a coupon on paper or embellishments when those things are on sale often enough anyway. I'd rather use the coupon on tools, like rubber stamps, dies for my cuttlebug, blades for my trimmer, etc...
Often times, I will wait til after a holiday or season and buy the papers once they've been marked down (when they are trying to get rid of the old stuff to make room for the new stuff). Last year after Christmas, I picked up christmas stuff that was marked way low. So, it's last years christmas stuff... in future years will anyone care what year it was from?
If I can't find a matched set that I like, then I will buy separately.
If I buy separately, then I wait til things go on sale. HL and M's will run paper by the sheet specials or sales on rub-ons, stickers, stamps, etc... I find it harder to buy by the piece than buying in matching sets, but thats just me.
My general rule of thumb is: I will only buy when things are on sale and I only buy things that I know I will use either on pages or cards. I don't buy things just because they are popular at the time or the latest and greatest thing on the market.