It can matter but it does not have to, and afaik it's not about column count, but rather row size, due to block accesses being expensive. Or used to be. It could be that solid state drives removes the need for concern alltogether in this regard. But I havn't read up on how they work, so that's idle spceulation.
So if you have a lot of data in one column which is rarely selected, you could gain speed by moving it to a separate table and keep a 1:1 relation between them. If it is included most or all of the time, you may actually get a performance decrease since there is a cost for joining tables.
If it is important, profile it.
