I’m brewing this week and ill be using SafAle S-04 yeast. My OG will be 1.066 and BeerSmith3 says i will need 1.3 (11.5g) packs of S-04 in a 5.13 gallon batch without making a starter.
I used dry yeast roughly once in the past 15 years, so i have virtually no experience with it. I have always read that one 11.5g pack is all that’s needed in a 5 gallon batch.
So, I’m looking for some opinions on this.
Thanks in advance!! 
As long as your yeast hasn’t been opened or damaged, then one pack is more than sufficient. Yeast calculators all tend to be overly conservative by about a factor of 2. So I mean, you probably only need 0.65 packs. Yes, I am very serious.
I’d round up to 1.5 or even 2 packs. 1 pack is good to a certain gravity in a certain volume at a certain temp. 1 pack is not good at any gravity in any volume at any temp.
I agree, I recently made a beer where I erroneously didn’t make a starter, but ALSO overshot my OG by a lot ending up around 1.060 instead of my estimated 1.050 and remembered why I try to never underpitch.
with dry yeast i can recall two occasions where I underpitched, one a long time ago, intentionally “to increase the esters” with munich classic - not good, harsh alcohols and problems.
another time I did what you are doing, thinking “close enough to 1 pack, it will be fine”. it could have been a great beer but it was a meh beer with the same harsh alcohols issue.
It’s not worth the risk of time and money put into a beer to chance anything.
I brew 2.5-3.0 gallon batches and always use less than 1 packet of yeast. Depending on what yeast I’m using and how big the packs are, my pitch is either 7.3 g. for an 11 g. pack or 7.6 g. for an 11.5 g. pack, dry with no starter. I’ve been doing it for years and have never had a problem. I always check the Brewers Friend pitch calculator and always have an excess of the required amount. So, I’m in complete agreement with D. Taylor that one 11.5 pack of S-04 is plenty for your 5.13 gallon batch.
All good advise and suggestions! Thank all!! 
Same here … unless I’m just trying to use up the yeast because it’s aging out; then I’ll throw the whole packet in.