I’m not gonna tell you to use any particular size, just as long as you stick to it. I personally prefer an 8px baseline grid because of it plays nicely with various typographic standards and has more divisors (1, 2, 4, 8px) vs. (1, 5, 10px), but I know 10px baseline grids are popular since all design applications ship with Shift+← as 10px. So use whatever floats your tugboat. You can use Nudg.it to change the default Shift+← distance to save yourself time. This does wonders when you’re designing in a style like Google Material Design that works off of an 8px grid.