If the grain is in focus then the negative will be too. Bear in mind that the definition of the grain will deteriorate the further out from the centre you will go but I focus on the centre and then stop down to F8-11 with my Nikkor 50mm and it will be sharp all over. This all depends on the quality of your lens too. Get the best that you can afford and it won't let you down. Nikon, Rodenstock or Schneider are the ones to aim for.
The best focussing aid I have ever seen is one made or at least marketed by a company called 'Peak' There may be the same focussing aid sold under different names but this one has a rocking eyepiece which you can use to focus on the image/negative from the very corners to see how for it is 'out'. I have one that has seen better days but still does the job.
They do occasionally turn up for sale 2nd hand so expect to pay around £80 or $120 for one in tip top condition.
If your enlarger has a movable head it may well be worth checking that it is truly vertical and parallel with the baseboard, by using a small spirit level on all the vertical and horizontal surfaces.