Most of the baitcasting reels I have seen have holes in the spool. How do you attach the line to these? I've always tied it to them with a slip knot. But is it acceptable to just stick the line through a couple holes and cross it over itself to hold it?

Yes.

I suppose you could and be perfectly safe. As for me, I've always ran the line through a couple holes and tied a slip knot. JMHO.

 

I use tweezers, and tie a double-base-wrap uni knot.  

 

27 minutes ago, bulldog1935 said:

I use tweezers, and tie a double-base-wrap uni knot.  

 

That's how I do it too if the spool has openings. If not, I put a loop in the end of my line and feed it through my line guide, the back feed the line through the loop to create a lasso (for the lack of a better term) that I can insert the spool through and cinch it down.

 

I do this with mono mainline, or backer line for braid, so it cinches down on the spool and doesn't slip. I've never had an instance of line slipping on the spool with this method.

Heck I have a couple of rods that don’t even have a knot, just a piece of black electric tape.

On smooth spools, I tie arbor knot, and secure the line with mylar/acrylic film tape.  

 

I tie a slip knot with an overhand stopper on every spool- spinning or casting- even if the spool is vented.  I'm sure the first time I don't is the next time that I hook a musky or striper on light tackle that spools me.  

Learn to tie the arbor knot. 
Tom

 

I didn't know what it was called until I looked it up, but I have been using the arbor knot.

 

Should I always run that through the holes or should I just wrap the spool with it?

I just use the Arbor Knot around the spool....no other anchoring other than the wraps of mono I use for backing on all my reels.

I started tying my braid direct to my spool, no backer. I tie it on with an arbor knot, cinch the knot tight to the spool. To prevent slippage, I basically tie two half hitches in alternating directions and it’s rock solid. 

my friend just runs the line thru and starts cranking.  no knot.  

Arbor knot and tape, casting and spinning

6 hours ago, MN Fisher said:

I just use the Arbor Knot around the spool....no other anchoring other than the wraps of mono I use for backing on all my reels.

Same here.