If a recipe does not call for a yeast starter, would it help or be harmful for me to add a yeast starter? The current brew I have going has a O.G. of 1.070.
It's almost always a good idea. Pitching the right amount of healthy, active yeast is one of the easiest and most important things you can do. This is especially true for high-gravity beer, which is harder for the yeast to grow in. There's a great and fairly concise discussion of these issues on Mr. Malty, as well as a calculator that tells you how many yeast cells you'd ideally want for a given starting gravity, volume, and whether it's an ale or lager strain.
The calculator says that for 5 gals of 1.070 OG, you'd need about 250 billion viable cells. A single vial/smack pack only has about 100 billion viable yeast cells at most (less if its old). You could pitch two packs, but that gets expensive. Alternatively, you could make a two-liter starter (2000ml of water + 200g of DME), pitch the yeast in that, and shake intermittently to grow 250 billion cells.
The conventional wisdom is to use starters, per the other answers here. I won't argue with that--there's certainly no harm in pitching a lot of healthy yeast.
That said, I've gotten very good attenuation at times just pitching dry yeast. My last brew was 1.077 and I pitched one packet of US-05 (5 gallon batch). So far it has fermented to 1.006. Pretty good. I wouldn't necessarily recommend doing this (I did it because that's all the yeast I had on hand) but if the rest of your technique is on point you don't necessarily need to worry too much.
Instead of going to the trouble of starters, I personally prefer to re-use yeast cakes. Ideally, immediately after siphoning off a batch I pitch fresh wort right on top of the cake. Alternately, I save 1 liter or so of the yeast and refrigerate until the next brew day.
This depends on liquid vs. dry yeast. As several other answers have said, with liquid yeast, yes you're better off using a starter if you don't want to buy 2-3 vials/smack packs. But dry yeast has a lot more viable cells than liquid. You could use a single dry yeast packet (e.g. US-05) for a 1.070 batch - it will get you near the 250 billion cells you need. It's definitely worth trying dry yeast to see if you like the results, because it's cheaper than liquid and easier than making a starter. This is especially true if you're using common strains like Chico/California. With good wort, sanitation and fermentation temp control, the results will be very similar and very good.