I was in a similar situation. Chronic hypertension, superimposed PE first pregnancy, controlled on meds second.
In my case, I was having a planned repeat section. My OB was willing to do that anytime between 39 and 40. She didn't actually give me a hard deadline, but I can read her fairly well and believe I would've had real pressure to schedule by 40. For convenience reasons (and because my nerves were, frankly, shot) I scheduled at 39+0. My OB would not schedule before 39 unless my BP rose (yep, MoD campaign, though they were implementing it reasonably and would have delivered earlier had it gone up). Having done this, I would probably be more comfortable than many here with pushing to 40, but not past your EDD. You really, really have to weigh the benefits of a few extra days against the possibility of pushing your luck. 25% is the figure for superimposed preeclampsia, but that doesn't include a rise in BP above baseline.
Controlled hypertension is a grey area. As far as I could tell (my son is 9 months now) there isn't adequate evidence on delivery timing in this scenario. There are OBs who deliver at 37 and ones who go to 41.
I had one serious and unexplained spike at 32 weeks (160/110 on the repeat after lying down for an NST) which earned me 24 hours in L&D. We watched my BP go back to normal that afternoon and it never went up like that again. We don't know why it happened. The thing with being chronic is that we know our ability to control BP is broken, or we wouldn't take the meds.
So, IME, an isolated spike isn't always treated the same for us. Not that it's waved off, but that it may not be a new problem, if you see what I mean. Just the same one we already had. That said, while I wouldn't take a single high reading in a chronic as evidence of any trend, we don't have great odds as a whole.
Ah ha--this came out after I delivered, perhaps it helps?http://www.ncbi.nlm.nih.gov/pubmed/21054760
(I can't get fulltext, but assuming that conclusion is supported, it would put me more firmly on the 39 weeks side.)