Cut baseboards vertically against the fence?

Agree 45 is better than 90. It is called a scarf cut and if done right, is barely noticeable when you put it together. You just have to make sure you make your cuts from the correct directions and on the same plane. That means practice on a scrap, getting 45 on the overlap from one direction and 45 on the underlap from the other direction. A little putty if necessary, along with a good fastening job, and you probably won't see it. 90's sometimes have a tendency to shrink or swell due to humidity and can be seen more than scarf cuts.

I would buy the best saw I can afford and definitely a larger blade type. you won't regret that for future usage. Who knows, next you will be doing crown molding along your ceilings....

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Cutting Baseboards With Miter Saw

You will get a better 45 degree cut, with the baseboard vertically against the fence. Get the 12" saw, you won't be disappointed, and in the future, you will find that it makes other cuts better. For example, cutting a 4X4 in half.

I'd go for the 12" saw for a couple reasons. It will be able to cut with ease making nicer cuts, it should be more powerful running at higher RPM's making nicer cuts, and if you decide down the road to get rid of it, selling it will be easier. If money is a concern you can find good tools on ebay used, or if you have a Cash Converters near you they have alot of good tools cheap as well. I just bought the 6 pc. DeWalt cordless set with an extra pc. included in great shape for $250.00 in the store, Home Dope Or Lowes (lesser Grade tools) that set runs over $600.00 I got 7 pcs. for $250.00

That is why they manufacture compond mitre saws. It will do what you want to.