But using sRGB for editing is not optimal since shooting in raw and converting to sRGB will actually result in clipped colors your camera can capture but that can't be contained in sRGB. ProPhoto RGB is the only colorspace that won't clip any colors your camera can capture.
To the OP, it's optimal to work in 16 bit ProPhoto RGB up to the point where you have to prepare the deliverable file. Then flatten, convert to sRGB (if that's what the lab demands, better to try to get a printer profile) then convert to 8 bit.
As Jeff has pointed out, 16 bit ProPhotoRGB is the optimal working space, at least for experienced users. However, you have to be careful in your editing, as you can easily push saturation to levels that can't be displayed on screen or printed and severe clipping can occur.
sRGB is not the native space of any printer, and those that request sRGB convert the image to the printer's working space. sRGB is not that bad of a choice for laser printers that print on photographic paper, as it contains most of the colors that can be printed. See the 3D gamut plot from Drycreek.com for the Fuji Frontier printer using Crystal Archive paper. Only a few high luminance yellows would be clipped from sRGB. However, there is a gamut mismatch and sRGB contains many colors that are out of the printer's gamut and these can be clipped.
It is much better to have a profile for the printer that you will be using, and the better labs can supply their profile. Costco does have custom profiles for their printers at Drycreek.com. Having a profile will allow you to soft proof in Photoshop and other more advanced photo editing software, and will allow you to choose a rendering intent, usually colorimetric but perceptual is useful in some cases. Perceptual rendering is not available for the sRGB space, even though Photoshop does not gray out that option. See the tutorial
on Drycreek for some basic information. The late much lamented Bruce Fraser
has posted a good tutorial on rendering intents. One pearl that I learned from reviewing the tutorial that saving JPEGs with the Optimized Baseline format will yield a smaller file in many cases and give better color. The Standard Baseline is the default in Photoshop.
If you prefer to work in 8 bit sRGB and use ACR, it is best to do all editing in ACR, which uses 16 bit internal processing. Editing an 8 bit file will result in data loss. If the gamut of your shot exceeds that of sRGB, you will see clipping on the ACR histogram. If there is no clipping, the use of a wider space has no advantage and can result in posterization of an 8 bit file.