You can safely use either. With a 40 proof (80%abv) alcohol, fresh does shorten the shelf life, significantly. So, when using fresh herbs to tincture, I just use a higher proof - think 'everclear'.
I'm a 'folk method' (as opposed to the 'scientific method') kind of gal. It's been proven over millenia to be safe and reliable, though there is always a little bit of variation, between batches, for several reasons. First and foremost, is the strength of the herbal constituents, from year to year, that are highly affected by weather and environment, for which there's often nothing to be done about, particularly if you're wildcrafting your herbs,
roots, and barks.
In tincturing, I almost always do at least a 1 quart batch. I start with a very clean jar - any kind of jar with a lid that seals well will do, but jars with measuring marks on the sides do make it a little easier. I fill the jar (*gently* packed) 2/3 - 3/4 full, if using fresh herbs, and only about half full, if using dried. Then, I start adding the alcohol, stirring gently, but well, to ensure the herbs are fully saturated, topping it all off, with
enough alcohol to fill the jar, run a butter knife through it, to release any air bubbles (you can simply use the knife to stir it, too). Seal it, label it with the date, the herbs, and the alcohol, including the proof.
You can then choose how you want to process it: 6 - 8weeks in dark & cool; a few weeks, in the sun; or (the fastest way) a few hours in the sous vide. Some people do their tinctures by the moon or the almanac. I haven't the time, patience or inclination, and I'm not convinced it makes a difference, other than a placebo effect.
After your tincture is strongly colored (& usually smells strongly of the herbs), you have more options. If you can be sure it won't be exposed to light, you can leave the spent herbs in the jar, and just tuck it in a dark pantry, and strain it, as you go - not my usual choice, for
medicinal, but I do use this in making extracts (vanilla
coffee, cocoa, etc - the process is the same). My preferred method is to strain, pressing out as much of the liquid as possible, then run it through a stainless steel coffee filter, and store it (well labeled with the same info from the first label), in a dark glass bottle (easier to pour from, than a jar) from which I'll decant into smaller dark glass bottles, as needed.
The reason I do it this way is that one large jar takes up less space in the cabinet than a bunch of little ones, but keeping the larger bottle in the cabinet, while using the smaller ones for each use protects the main bottle from spills and accidental contamination.