No, it is not necessary. People have been doing this for many years without pre-acidifying with much success. However, let me explain why you may want to do it, and you can decide for yourself if you want to bother.
First, many spoilage bacteria survive and flourish at pH 4.8 and above. By dropping the wort pH to 4.5, you stop bacteria responsible for sour mash stink in their tracks. Clostridium bacteria, enteric bacteria... the things that make a sour mash reek, can creep into a kettle sour if you don't practice good sanitation. Use a pure lacto culture and practice good sanitation, and it won't be an issue. Sour with grain? It's in your best interest to drop the pH to inhibit these bacteria.
Second reason, lactic acid bacteria have much proteolytic activity at pH 4.8 and above. This means they break down proteins in the wort, which also contribute to head retention. Have you had a sour beer that was fizzy without any head retention like a fountain soda? They didn't drop the pH, and the LABs broke down the proteins that aid in head retention. (Pro tip - dry hop your sours, and it really aids in head retention.)
Since you don't have a pH meter, a good rule of thumb is that 1 ml of 88% lactic acid will drop the pH of your wort by 0.1 point. Use distilled water, and DME should give you a wort of around pH 5.4 or so. So add 10 ml 88% lactic acid, and you're in the ballpark. This is just an approximation, but it'll get you close. If you have hard water, just double the lactic acid and you'll be ok. Remember that pH is a log scale, so to go from pH 5.5 to 5.4 takes a lot less lactic acid than to go from 4.5 to 4.4. So you have a lot of margin of error.
Hopefully this helps. Pasteurize your wort, use goodbelly as a pure lacto source, and just wing it. It'll turn out great.