Good question and no it does not matter, not in the slightest. People tend to make a big deal out of glass cockpits or six packs, but at the end of the day they really all display the exact same things. In fact, if you look closely at a glass cockpit you will notice that the instruments are in the exact same configuration as a glass cockpit has them.
Training on “steam gauges” will in no way affect your ability to get into ATP or get hired by an airline. In fact, I highly doubt that anybody will ever even ask you what type of cockpit you trained in.