I don't mean this to sound snippish or snide, but the only reasonable answer is really ...
All of them.
Or perhaps put another way ...
All the ones you don't recognize, and perhaps some of those that you do.
And I know, that's not really a reasonable answer at all. Let's look at what you can tell from the task list, and what you might consider looking at if you suspect a problem.
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"Be wary of" is kind of a vague notion, because in reality, almost all programs can cause problems at one time or another - including those that are a part of Windows. If your system is working well and you have no problems, there's typically nothing I would have you look at in task manager specifically. As long as you're running up to date anti-spyware and anti-virus software, and just generally following all the recommendations on how to stay safe, it's probably not worth your time to worry about it.
However, if you have reason to suspect a problem, there are a few things you might look at.
Examine Process Names: The Processes tab on Task Manager, and my preferred replacement for it, process explorer, both list the programs running on your machine. (Be sure to check Show Processes from all users in Task Manager.) Those will be program names like svchost.exe or taskmgr.exe - names that probably mean nothing to most users.
Google each one that you don't recognize. Typically on the first page of results will include links to various sites that describe what that program does, or why it exists. Most, if not all, will have reasonable explanations that will make sense for your machine. For example I have a process "atiptaxx.exe" which turns out to be related to the ATI graphics card in my machine. Each machine will probably have a different set of programs running depending on its configuration or what additional software you have installed.
Examine CPU Usage: In both Task Manager and Process Explorer you can click on the "CPU" column header and the programs will list the processes running on your machine in order of who's using the computer's processor (or CPU) the most. Most of the, if your system is operating normally and not doing anything, the "System Idle Process" should be taking most of your computer's time. If it's something else, you can then see what that is (perhaps by Googling it, as above), or simply realizing that a specific program on your computer is actually doing something.
Examine Memory Usage: Once again in both Task Manager and Process Explorer you can click on the "Mem Usage" (or "Working Set" in Process Explorer) column header and see who's using the most of your computer's RAM. This will vary widely depending on what software you are running, and what it's been doing. In Process Explorer you can also click on the "Virtual Size" column header to see who's using the most total virtual memory (both in RAM, and swapped out to disk). Programs that are using excessive amounts of memory can often degrade your systems performance.
In general, though, the best way to keep your system safe is to make sure you're following those stay safe recommendations. There's rarely a need to watch programs in Task Manager, or more specifically, there's rarely a reason to "be wary" of programs in Task manager, unless you're suspecting a problem that hasn't been identified through other means.
