Most simple styles may be knitted top down if one prefers.  There are some advantages to doing so. One is that the  ribbing is at the end of the piece rather than the beginning. If  hand latching ribbing, many find it easier to do this way. And  whether hand latching or using a ribber, controlling the look  and stretch of the lower edge of the ribbing can be easier  when it is the bind off instead of the cast on. Plus it's just fun  to know how to turn a project upside down. 

Another great thing about top down knitting is trading  increases for decreases. An example is a long sleeve. If  knitted from the wrist up, there will be many increases.  Simple increases are nice and fast but sloppy. Full fashioned  increases are nice and neat but a little more time consuming  and a little more challenging, especially to those with less  than perfect vision. If we work the same shape of sleeve  from the top down, it will be shaped by means of decreases  rather than increases. Full fashioned decreases make a nice  neat edge and are a little bit faster and easier than full  fashioned increases. 

Top down knitting also gives more opportunities for knitting  pieces right onto each other. For example, if a sweater front  and back are already complete, we can seam the shoulder,  then hang the armhole on the machine and knit the sleeve  right onto it. No hand seaming will be necessary! I have a  video called "Knitting the Sleeve Right into the Armhole of a  Top Down Sweater" to help you with this. This may be done  even with shaped set in sleeve styles, though of course it is a  ittle more involved than with a dropped or modified dropped  shoulder design 
Most basic styles such as those in my book: The Answer  Lady's Machine Knitting Notebook may be worked either  bottom up or top down. Here's how one would take a basic  sweater front pattern and knit it top down instead of bottom  up. Suppose the instructions read: Cast on 100 stitches. Knit  12 rows of rib. Knit straight to row 100. Bind off 10 stitches  at the beginning of the next 2 rows. Knit straight to row 140.  Bind off the center 10 stitches. These are the bottom of the  neckline. Place the needles on the side away from the  carriage in hold. Knitting on the remaining needles, decrease  1 stitch at the neck edge every other row 10 times. Knit a  final 6 rows. Bind off the shoulder stitches.  We now do a little bit of math. Subtracting the stitches  bound off at the armhole tells us that there should be 80  stitches across the top of the sweater just below the  neckline. Subtracting those stitches included in the neckline  tells us that the top of each shoulder consists of 25 stitches.  Each batch of 25 should be at the outer edge of the span of  80. Adding up the rows we discover that the final row is 166,  even though the pattern doesn't say so. Since we know that  there were 100 rows before the armhole shaping, subtracting  100 from 166 tells us the armhole depth is 66 rows. The total row count above the ribbing is the same whether knitted bottom up or top down.