My answer supplements Sobachatina's.
It's a matter of risk, and the issue is that what you're risking is a horrible death.
If you merely wash the cans, fill them with a hot mixture, and then put them in a hot water bath (or pressure canner), then you are probably killing 95% of the bacteria and fungi that you'd have killed if you also sterilized the cans. The problem becomes that if what you're missing is even a single botulism spore, then you can produce a can which, for days or weeks, looks OK, tastes OK, and will kill you.
So it's a matter of how much risk you're willing to tolerate. What you're canning also makes a significant difference; high-acid or high-salt foods (jam is generally high-acid) are significantly lower risk than, say, canned tomatoes.
If you are going to skip a step, sterilizing the cans is the step to skip, rather than the water bath. The reason is that the air in the can has bacteria and fungi in it, so you need to sterilize after filling the can (or jar).