Opportunity is missed by most people because it is dressed in overalls and looks like work. - Thomas Edison
Ideally, you want a single strong (all the time) on everything. Sometimes, as a band aid, you will hit twice for something but in general, 1 hit always on all it (the goal). So, all in all, I think everyone is talking one hit.
This is a great thread/great question. Screenprinter123, we think alike. I actually just ordered up a small grip of yardage in the 270T for the exact reasons you laid out. We have a huge gap in our upper meshes as our lineup (excluding specialty mesh) is: 135-150-180-225-330. That's a huge gap between the 225 and 330 and that 270 has really good numbers on paper as well as a small thread size despite it's "T" designation. For reference the mesh goes SS (super crazy thin), S, T (standard), HD and then they'll use thread micron suffixes for ones that fall in between.I have to give a 2pt answer since 90% of our sim pro is wb/dc nowadays. For plastisol over a base I always, always, always want less ink deposit and finest possible lpi. I am in Dan's camp in that you must have enough total ink across the image to use higher mesh appropriately. That said, for sim pro over a plasti base (which I avoid like the freaking plague) I may even prefer the next mesh up from a 330/30 S, less is just better here for WOW printing. Going onto the shirt or onto a DC UB, more is OK there. Now, wb/dc is another animal. We need penetration and we need a minimum amount of it to ensure a long lasting, wash fast print is made. You run a risk here of doing a gorgeous job of saturating the very top strata of the fabric, patting yourself on the back and then sending out a print that will fibrilate and fade in very few launderings. This would lead one to get spooked and run lower LPI on more open mesh. We do run 225/40 for larger spot color fill areas and very basic sim pro jobs using WB/DC. It's easier on press, penetrates in one stroke when you need it to and looks pretty darn good for a lot of jobs, think of your punk/metal band type of art with more basic halftone blends and fades, most of that art is fine at 45-50lpi. The 225/40 can stand on it's own with saturation for spots/halftones on one screen. However, the 330/30 needs buildup, overprinting and enough overall colors smooshing it all down to saturate most fabrics. Dan's comment on print color count is extra accurate in this case as is Pierre's mention of fabric weave. There's no hard and fast.