Yep - even though I refused to purchase one, I would recommend a harness if your boston is a puller (and I think the majority of them are). I didn't want one because I didn't want Jake to have to wear it around the house all the time and I also wasn't wild about having to put it on him every time I take him out. It took a lot of work, but I eventually trained him to not pull so much and to keep the leash loose, which was a blessing.
I did notice that while he was pulling, he coughed and choked quite a bit, so the tension distribution of a properly-fitted harness would have done him a world of good...if it wasn't for my stubbornness.
