Scraping Caramel for Flan America's Test Kitchen proposes the following recipe as their &quot;Perfect Latin Flan&quot; (pg. 785, The Complete America's Test Kitchen Cookbook 2001-2016):

This recipe should be made at least 1 day before serving. We recommend
an 8.5 x 4.5 inch loaf plan for this recipe.
4 2/3 oz sugar
1/4 cup water + 2 tablespoons warm tap water
2 large eggs + 5 large yolks
14 oz sweetened condensed milk
12 oz evaporated milk
1/2 cup whole milk
1 1/2 tablespoons vanilla extract
1/2 teaspoon salt

Stir together sugar and 1/4 cup water in medium heavy saucepan until sugar is completely moistened. Bring to boil over medium-high
heat, 3-5 minutes and cook without stirring until mixture begins to
turn golden, another 1 to 2 minutes. Gently swirling pan, continue to
cook until sugar is the color of peanut butter, 1 to 2 minutes. Remove
from heat and swirl pan until sugar is reddish-amber and fragnant,
15-20 seconds. Carefully swirl in 2 tablespoons warm tap water until
incorporated; mixture will bubble and steam. Pour caramel into 8.5 x
4.5 inch loaf pan; do not scrape out saucepan. Set loaf pan aside.

Adjust oven rack to middle position and heat oven to 300 degrees. Line bottom of 13 x 9 baking pan with dish towel, folding towel to fit
smoothly and set aside. Bring 2 quarts water to boil.

Whisk eggs and yolks until combined. Add sweetened condensed milk, evaporated milk, whole milk, vanilla, and salt, and whisk until
incorporated. Strain mixture through fine-mesh strainer into prepared
loaf pan.

Cover loaf pan tightly with aluminum foil and place in prepared baking pan. Place baking pan in oven and carefully pour boiling water
into pan. Bake until center of custard jiggles slightly when shaken
and custard registers 180 degrees, 1.25 to 1.5 hours. Remove foil and
leave custard in water bath until loaf pan has cooled to room
temperature. Wrap loaf pan tightly with plastic wrap and chill
overnight or up to 4 days.

To unmold, slide paring knife around edges of pan. Invert serving platter on top of pan and turn pan and platter over. When flan is
released, remove loaf pan. Use rubber spatula to scrape residual
caramel onto flan. Slice and serve. Leftover flan may be covered
loosely and refrigerated for up to 4 days.



It would seem like the amount of caramel left in the pan if you don't scrape would vary based on the size of the saucepan you use. Why would you not scrape the remaining caramel into the pan? Would it affect the texture/flavor?
(note : I'd have posted this as a comment because I don't have personal experience w/ this, but people keep complaining when I do as this might have enough information to be considered an answer)
I don't know about this case, but in other times when you're cooking down sugar there's always a fear of 'seed crystals' getting into the sugar (causing it to become grainy).
A little searching seems to support that theory.  There were many websites talking about seed crystals, but I'm going to quote the one from Fine Cooking (The Science of Caramel), as they specifically mention problems with the sides of the pan:
http://www.finecooking.com/item/60729/the-science-of-caramel :

What can go wrong when making caramel?
The caramel turns grainy
The biggest drawback to the wet method is that the sugar tends to recrystallize more easily than it does with the dry method. When the sugar and water boil, sugar syrup may splash onto the wall of the pot, where it evaporates quickly and forms back into sugar crystals. If even one of these crystals falls back into the syrup, it can seed a chain reaction, turning the clear syrup opaque and grainy. Should this happen, you can remove it from the heat, add a few tablespoons of water, return it to the heat, and stir until the crystals dissolve before continuing on. That said, it's better to keep recrystallization from occurring in the first place, so here are a few ways to prevent it: ...

See the article at Fine Cooking for advice on how to prevent seed crystals and other information on caramel.