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In the past, some yeast manufacturers (namely Wyeast) did not recommend making a starter for mixed cultures. Reasons varied from "throwing off the ratio of microbes" to "creating acetic acid". In regards to the first concern, throwing off the ratio of microbes will likely not be a real concern. It is possible that this will change the profile of the fermentation (for example the beer might turn out more sour because the lactic acid bacteria were allowed to grow more cells in the starter), but if treated properly all of the microbes will have a higher cell count after a starter is made, and the yeast (both ''Brettanomyces'' and ''Saccharomyces'') will be more viable after a starter. Pitching a second culture of fresh ale yeast isn't necessary with mixed cultures that have their own ''S. cerevisiae'', and making a starter ensures the cell count is high enough. However, some mixed cultures might not come with ''Saccharomyces'' out of the package, or in the case of a [[Mixed_Fermentation#Reusing_a_Sour_Yeast_Cake|re-using a sour yeast cake]] the ''Saccharomyces'' might be dead. In such cases the brewer may choose to pitch fresh ''Saccharomyces'' separately (see [[Mixed_Fermentation#Reusing_a_Sour_Yeast_Cake|re-using a sour yeast cake]]). Oxygen does not inhibit lactic acid bacteria in general, so this is generally not a concern.
In regards to the concern about oxygen, oxygen does encourage acetic acid production in ''Brettanomyces'' (and some species of ''Lactobacillus'' such as ''L. plantarum''). However, a starter can be decanted to remove the starter beer if it smells highly acetic. Some brewers have reported not decanting the starter at all, and the small amount of acetic acid present doesn't make a large impact on the final beer. Therefore, in general, it is good to use aluminum foil to cover the starter instead of using an airlock. If the starter is going to be stored for longer than 2 or 3 weeks, then use an airlock to limit the acetic acid production and store it at refrigerator temperatures (long-term exposure to acetic acid and low pH will lower the vitality of ''Brettanomyces'', ''Saccharomyces'', and lactic acid bacteria). The viability of the microbes will get worse the longer the starter is stored (some strains of ''Brettanomyces'' , saison yeast, and lactic acid bacteria might survive longer than regular brewer's yeast; a starter that is 6+ months old should be treated like a [[Mixed_Fermentation#Reusing_a_Sour_Yeast_Cake|sour beer yeast cake]]), so if it is stored for more than 2 or 3 months, then a new starter should be made to make sure the cell count is high at pitching time. Letting the starter grow for 3-6 days with light agitation (either frequent manual stirring, a stirplate on a low setting, or an orbital shaker) before pitching is a good general rule of thumb.
If the package is fresh, then a starter is generally not required. If the package is nearing expiration or expired and buying a new package is not an option, or if the batch of beer is much larger than what the package is intended for, then we recommend making a starter for all mixed cultures (including Wyeast Roeselare). In general, we recommend a 500-1000 mL starter (~1.040 OG with DME) for a 5 gallon batch. Run a stir plate on the lowest setting, or use occasional manual stirring with no automatic stirring. If the package contains ''Brettanomyces'', then run the starter for 4-6 days; if the package does not contain ''Brettanomyces'' then run the starter for 24-48 hours. Do not use hops in the starter unless inhibiting the lactic acid bacteria is desired. One may want to have an anaerobic starter in the case that the lactic acid bacteria are especially weak, and the brewer wants to inhibit the yeast growth in the starter. Remember though that yeast needs oxygen in order to grow viable cells <ref>[https://www.facebook.com/groups/MilkTheFunk/permalink/1660420660652808/?comment_id=1660436453984562&comment_tracking=%7B%22tn%22%3A%22R8%22%7D Pat Tkacz and Justin Amaral. Milk The Funk Facebook group. 04/21/2017.]</ref>. Pitching the entire starter should be fine, even if there is some considerable acetic acid production (see [[Brettanomyces#Starter_Information|''Brettanomyces'' starters]]), although it can also be chilled and the starter beer decanted if desired.