Not if you're just doing basic rhythm interpretation. With the caveat, that sometimes it's just easier to do a 12-lead, get a dozen views at once, and compare them, if you're having trouble identifying P waves, looking for some of the VT criteria, deciding if the QRS is wide, etc. For me, at least, anything weird is pretty much getting a 12-lead anyway unless the patient is near-death (*not just unstable).
It's probably better to put them over conductive tissue, i.e. not on bony prominences. If you place them more centrally you'll get larger amplitude complexes, but you may get more respiratory artifact (although the filters should take care of most of this).
Filter settings may vary slightly between monitors. So if you put two monitors from different manufacturers next to the patient, hooked the cables up to the same electrodes in the same position and looked at tracings, there might be small differences machine to machine. They should look pretty much the same though.
But if all you're doing is very basic interpretation, pretty much anywhere you place the electrodes that gives a big enough complex to interpret is sufficient.
Obvious points:
* Yes, they probably shouldn't be randomly placed. It would likely work, but it sounds a lot more professional to say I looked at lead I, II, III, aVR, aVF, aVL, versus lead "just over the liver to just under the suprasternal notch".
* It's often easier to interpret the rhythm from a 12-lead. It does matter where the electrodes are placed here.