Drawing a comparator op-amp in Multisim
I'm pretty sure it's quite common to omit an op-amps power connections when doing a circuit schematic, it's just for clarity I think - it's presumed you know they're supposed to be there, it keeps diagram complexity down and makes other connections easier to visualise.

I'm not familiar with Multisim, but I'd imagine - If the option to connect the power rails is there, you should probably hook them up, in theory the op-amps will not function correctly if they don't have the power hooked up, but this may not be the case in the sim-world. 

You can always test both - if the simulation still works without the power connection - it may be that it assumes the nominal power supply is connected. The connection may be more applicable for testing power supplies outside the nominal range.

Also - The de-facto circuit simulator that most people seem to use is SPICE, as it's quite common, it has a good community base and there's loads of help and tutorials available for it.   

Just my thoughts on the matter, some of it may be helpful :)