Edward Longacre asks this question in his article on Grant. He writes:
That Grant drank occasionally while on duty is a matter of record, as is the fact that on more than a few occasions he drank until intoxicated, stuporous, and violently ill. More difficult to gauge is whether this habit hampered his ability to command or (as some observers contend) propelled him toward military success even as it marked him as a failure in civilian life.
He goes on to assess the type of Grant’s drinking:
Grant did not fit the stereotype of the falling-down drunk. He drank at irregular intervals, in varying quantities, and with differing results. At times he imbibed moderately, with little or no noticeable effect, and he was capable of refusing a drink, explaining that alcohol brought him nothing but trouble. Even so, he was, in the clinical sense of the term, an alcoholic. On more than a few occasions he drank long and hard, unable to stop short of unconsciousness or some form of intervention -- the outbreak of attention-demanding military operations, or the ability of relatives or staff officers to limit his access to liquor.
Longacre says that Grant’s drinking actually resembles binge drinking. He also appeared to be able to control it at times:
Although historians continue to debate the extent and the effectiveness of his efforts, Grant’s adjutant general, John A. Rawlins, a zealous cold-water man from Grant’s home state of Illinois, strove to cover up his boss’s indiscretions and swear him to abstinence. Grant appears to have given in to his urge to imbibe only on those occasions when Rawlins was not on hand to ensure his sobriety. Nor did Grant drink when Julia and the children visited him in the field, rescuing him from lassitude and loneliness.
His final conclusion was that alcohol didn’t detract from Grant’s Civil War performance:
Given the high visibility his position attracted after 1861, Grant’s dependence on alcohol might well have threatened his continuance in command. Instead, through a combination of factors -- his determination to abstain when military operations were in progress; a moral strength based on religious values that has escaped the notice of many historians; and the support of relatives, subordinates, and political backers, chief among them Abraham Lincoln -- Grant persevered to play a critical role in ending America’s costliest war and restoring the Republic.