Tyre stretching is putting a narrow tyre than standard on a rim.
They will fit.
Rolling radius might be put out a little bit depending on profile of the tyre.
Normally done with an adjustment in tyre profile so that wheels will fit under guards without fouling, or to maintain a certain rolling radius (so speedo is accurate).
155 on a 5" rim is nothing major though, I have 165s on my 7" rims with out issue.
Basically a narrower tyre will have less surface area with which it can grab the road with. So slightly less grip/traction than with a wider tyre.
The other issue is the load rating of the tyres, in the glove box there is a minimum tyre load rating on the recommended tyre pressure placard. You should make sure that the tyres meet the minimum requirements (more of a defect issue as micras weigh half of eff all)