Do we? Don't we? Should we? Shouldn't we?
On the whole, I think go for it. Not with anything precious or very old but alas, life being what it is, I am not likely to own anything like that.
I do often have to battle the nagging voice in the back of my head that says "Should you even be altering this? It is vintage." (the voice says vintage in a very Ab Fab sort of way) Let's not be precious though. Just because something happens to be more than 15 years old is no real reason to preserve it untouched. Fashions and hemlines change and a piece of clothing is far better off being altered so that it fits and is wearable rather than hanging on a rail unloved for all time.
If it was an expensive purchase or difficult to alter, I may consider sending it to someone who is slightly less cackhanded with a sewing machine than I am but with things that aren't quite so old, I don't feel quite so much pressure. I (eventually) have a crack at the alterations myself. Here's one I did last year.
I picked up two very nice Welsh tweed skirts from Oxfam months ago. Rather a snip at £6.99, thought I. But, oh, they were a hideously unflattering length. Look, see for yourself (and ignore the Mitch Benn tshirt. I only realised at the last minute that I should probably take a photo before starting the alterations):
Turns out it wasn't so very difficult to make them look good after all. Measure, pin, hack and hem. Bish, bash, bosh. They're both beautifully made, so it was just a case of trying to do it neatly enough to do them justice.
Additional bonus - a lot of offcuts! I think I shall attempt making a clutch bag. Watch this space. I may get round to it by...ooh...September?
And what do they look like now? Well you can only see one because I haven't photographed them both, but here is the darker of the two. I'm very pleased with the results.
Shirt - Zara, via charity shop
Skirt - vintage, via Oxfam
Cricket jumper - Oxfam. No idea if it's vintage or not, it's just cute
Brogues - Dune, via TK Maxx
Coat - Kookai, via car boot sale
Gloves - Accessorize