Well, I hate telling anyone that they are wrong, but those offsets are not the correct spec for the slingshot
Base Model
F 17X7 +42 MM 5 on 4 ½ BC (114.3mm)
R 18X9.5 +45 MM
S/L Models
F 18X7.5 +42 MM
R 20X9 +45 MM
All models use the same +45mm offset on the rear tire. The one they are using is off center by 3mm. Truthfully, it's a small amount, but it is off centered none the less. I'm not even sure you can find wheel spacers that small to center it. If they say they have used them with no issues, then it's up to you to make the decision.
As for the front wheels having a 10 mm difference will bring the wheel 10mm closer to the body/suspension components. As long as you have no fitment issues, you could probably be fine. You would not want to go to a larger offset if you were planning on using a trailer as most of the slingshot trailers are awfully tight as it is with the stock width. So if you don't like the narrower stance, you could use a wheel spacer to bring out the front wheels that 10mm so it's back to stock specs.
Hope this helps (and I got it right)