As long as the pH is under 3.8, you're probably okay so long as you keep your SO2 levels up. Above 3.8 is where things start getting really dodgy since it becomes a very hospitable environment for spoilage organisms. Dropping the pH will give you greater microbial stability, but it might throw your wine out of balance. If you really want to drop the pH a bit, I would do some bench trials to see whether adding acid is an option or not... It might taste pretty balanced now, but even a small adjustment might make it go over the top in terms of tartness.
If you're going to add acid, use tartaric since it will have the greatest effect on pH, although some may precipitate out again as tartrate crystals... which means that your pH may change slightly again after these tartrates precipitate.
You can easily do a few 100ml samples with various degrees of acidification, say 0.25g/l, 0.5g/l, and 1g/l and then test for pH and taste them to see whether or not they taste out of whack with the added acid. Then you can simply make the call as to whether or not the pH changes that occur are significant enough (and taste balanced enough) to make any acid adjustments to the whole batch.
If you don't have a scale capable of weighing out small quantities, you can make an approximate 5% tartaric acid solution by dissolving 1 tsp (approx. 5g.) of tartaric in 100ml of water and then adding the appropriate quantity to each sample. Each ml of 5% solution will contain 0.05g, so to add 0.25g/l to a 100ml sample you would require 0.5ml... 1ml for 0.5g/l and 2ml for 1g/l. I find that a syringe is really handy for measuring out small quantities of solution. You'll probably want to check the math on this as I'm writing it in a bit of a hurry, but I'm pretty sure it's correct.
Anyways, sorry for the long, rambling response... hope it helps.