It really depends on the length of time recommended. Due to the usual long length (>4 hours) of slow-cooker recipe cook times most elevation differences would be negligible. However if a recipe calls for a cook time of less than 4 hours you may see some foods not cooking as usual. Legumes might especially be sensitive to this as they take a very long time to cook completely. Below is a table from Wikipedia on elevation vs water boiling temperatures.
Altitude, m Boiling point of water, °C
(0ft) 100 (212°F)
(984.25ft) 99.1 (210.3°F)
(1968.5ft) 98.1 (208.5°F)
(3280.8ft) 96.8 (206.2°F)
(6561.68ft) 93.3 (199.9°F)
(13123.36ft) 87.3 (189.1°F)
(19685.04ft) 81.3 (178.3°F)
(26246.72ft) 75.5 (167.9°F)
You may need to experiment with longer cook times for foods that you are having trouble with.
Ok, from the clarification in the comments:
I'd say that roughly speaking, "high" cooks twice as fast as "low". This is a very rough estimate, and also depends on the slow cooker a lot (they vary in temperature...). So, each hour you spend on high is like spending two hours on low. You can use that to figure when to turn your slow cooker up so its ready on time.
Example: You have 7 hours until dinner. Recipe says 9 on low. So you do 2 hours on high and 5 hours on low. The 2 hours on high count for 4 hours of cooking on low, so you have the 9 hours the recipe called for. And 5 and 2 are the 7 actual hours you have until dinner.
Given how approximate the conversion is, you should probably plan to have it ready a bit before time, and then turn it down to "keep warm". Note that cooking will continue on "keep warm", especially as it'll take a while for the food to cool. Keep warm should be around 150°F, which will keep the food safe to eat (its out of the danger zone), but will actually continue cooking, just very slowly.
Also, you should check on when its done by visual clues (as much as possible), and (as quickly as possible) check the texture. You want to keep the lid closed as much as possible, opening it will slow it down a lot. Check no more than once per hour.
If you have a thermometer you can leave in the meat, you could use its internal temperature. My newest slow cooker has a temperature probe, but most don't, so that's probably not an option. Though depending, you may be able to stick a probe thermometer through one of the vent holes in the lid.
Best Answer
I don't know if there's an 'official' way of doing this, but I've heard a few different techniques (eg, putting the crock together the night before, then fridging it).
Personally, if I'm going to prep things in advance, I try to take advantage of the extra time, such as by giving the meat a dry-rub.
In theory you can mix the meat and vegetables (as they'll all be cooked to sufficient temperatures to make the meat safe; the problems are when you cook the vegetables for less time) ... but I find that makes it difficult to specifically layer the crock the way that I want to. (I typically put a carrots across the bottom to lift the meat a little off the bottom of the crock).
When it comes time to put everything into the crock, I start it on high for a little while so it'll come up to temperature in a reasonable time, and then switch it over to warm before leaving it to work. (it's possible that the new programmable crockpots do this automatically with 'fuzzy logic' ... I've never tested it specifically).
...
So, to summarize:
If you overslep or otherwise need to get out the door quickly ... put the liquid in the microwave to heat up (or stovetop) while you assemble the stuff in the crock, and assemble it while the crock is on low. (you won't burn yourself right away, as it takes a few minutes to warm up). Once the liquid is warm (you don't want it at a boil ... we're looking for near our target 130-140°F / 57-60°C; a couple of degrees over if you have the time), pour it in slowly, working your way around the outer edge of the crock. Drop on the lid, and then go to step #9.