Can I titrate by taking 1/4 of 5 mg Proscar twice a week, then three times a week, and so on?

You are certainly able to take this medication in doses and see what works best for your body. If you find that you might have side effects you will know within the first two weeks. If this should happen get off of it. If not I would increase and see how you feel. 

This is a reasonable plan. Generally making these changes in dose every 1-2 months makes the most sense. 

This is a great technique to use if you are worried about side effects.  I recommend it often at our clinic.  It is also a great idea if you have had side effects on finasteride before and want to try it again.
I recommend moving up the dose slowly.  If for some reason, you get side effects at a higher dose, you can simply lower it again.  The medication should still be effective even at the lower dose at helping to preserve hair.