I really think it depends what type of stripe your going to do and what your current car "mod status" is. On a bone stock car, it will probably look more retro than on a modded car. And by modded, I mean, upgraded wheels, side skirts, and potential painted calipers or rims. A biggie is the style of the stripe too.
If you just want to plant PORSCHE on the side in fat letters, then it'll look old fashioned. But if you make smart use of the side stripe to break up boring lines then I think you give the car a wider looking stance. I personally prefer a fat stripe with a thin one extending from the front of the car to somewhere I'm the middle... Or from the back to somewhere in the middle.
Lastly, with existing upgrades, a side stripe is a great way to tie them together. Like brake calipers and rim color. Even breaking up the the side skirts from the body of the car to give it an aggressive feel (works better on metallic colored cars). I mean, when you look at 911s and gt3s, porsche ties in the side stripe to subtle features of the car to make it the otherwise insignificant addition, noticeable. If you plan on adding a side stripe, I think you should do the same... Pick a unique design that's tailored to your upgrades. Just my 2 cents.
Here is a classic, "retro" side stripe
This one, IMHO, makes better use of the car pre-existing upgrades to tie the whole body together
