Firstly,I wiill say openly and honestly that i am new to reloading,and that my own experience is some what limited,but i would like to tell you of my own person experience neverthless.
When i started reloading for my .243 with my once fired federal brass, i did not trim the cases,i measured and all were within 10thou of each other.
The only reason i did not trim was because i did not have a case trimming tool at the time,however,having been shown the basics from a good friend,and reloading in his company was assured that they would be fine and safe.
I seated my bullets to the length of my factory ammunition,which at the time was federal,again imo worked very well in my rifle,and the first batch of reloads went through the same hole at 100yds, by far the best group I have ever had.
I then went on to trim a batch of cases,and all were different to that of the trimmed batch in terms of accuracy, and am only now getting some more batches consitently after trimming.
In any case i bought the lee case trimmer and case holder,and fixed it to a cheap coordless drill from aldi,and thats how i trim my cases.
Hope this helps.