I firmly believe there is nothing wrong with mocking up a muslin/toile. And that decision is completely up to you, so don't let anyone else tell you otherwise. Also, if you are totally against muslins, don't bother with one, because I get where you are coming from too. Either way, here's a few things to keep in mind.
A previous muslin I've made - post is here
If you are going to make a muslin, consider only making up the bodice because let's be honest, the skirt section on Simplicity 1880 is most likely, 9 times out of 10, going to fit you. I say that because the skirt is a flared 1/2 to 3/4 looking circle skirt and those fit nearly everyone bearing in mind that you've picked the right size for your waist and hip and double checked that with the finished waist measurement that is found on the pattern piece itself, which is pattern piece #15 btw.
Before you go cutting out a muslin straight from the pattern, also consider doing lengthen and shorten adjustments. If you know you're long waisted, make that adjustment. Hold up the skirt section to yourself and have a gander in the mirror and check to see if its falling to a comfortable length for you. Do all this with some elastic tied around your waist so that you know where your waist is hitting and you can double check that against the pattern which has the waist clearly marked on piece #15.
If you've made up several Simplicity patterns and you have an idea of where something is not going to fit - for me that's always in the sleeve and armscye - do the alteration you usually do before you make up your muslin. It's more than likely, in fact 99.5% more likely that you'll have to make the same alteration again that you've made on countless other patterns, so just go for it and do the alteration before the muslin. This will help with future makes in that you might find that you're able to wean yourself away from muslins and just make the alterations you know you're going to need.
If you've made a muslin and you've found that you need to make all your usual adjustments and alterations plus you're not quite sure about this or that add more seam allowance to the this or that area and from there you can adjust the final garment using the seam allowance rather than making up one more muslin.
So how's that for some tips? What are some of your thoughts on making a muslin? If you've made a muslin and need some fitting help and/or advice, take some photos (please wear your muslin in the photos) and put it in the flickr group and we can all help. I'll be back tomorrow with more on alterations and where to find some great tips and tricks on what to do for a particular fitting problem.
Happy Muslin Making!