There are multiple options. First, note, that all computations are in JavaScript, not in the markup.

Option 1 You implement the display of your {item} (which is currently only the JSON object), into a dedicated component AccountListItem. And implement in there the logic that @POZ suggested.

Something along the lines of:

import {LightningElement, api} from 'lwc';

export default class AccountListItem extends LightningElement {
   @api index;
   @api item;

   get isMod5() {
     return this.index % 5 == 0;
   }
}
 

Option 2 You use a wired function instead of a wired property. And then do some fun stuff with a changed data model. Although this is hacky.

import {LightningElement, wire, track} from 'lwc';
import getAccounts from '@salesforce/apex/TestController.getAccounts';

export default class AccountList extends LightningElement {

    @wire(getAccounts)
    wiredAccounts({ error, data}) {
       if (error) {
         // do error handling, please
       } else if (data) {
         this.accounts = data.map((acct, index) => {
               return {
                  acct: acct,
                  isMod5: index % 5 == 0 ? true : false
               }
           })
       }
    }

There are multiple options. First, note, that all computations are in JavaScript, not in the markup.

Option 1 You implement the display of your {item} (which is currently only the JSON object), into a dedicated component AccountListItem. And implement in there the logic that @POZ suggested.

Something along the lines of:

import {LightningElement, api} from 'lwc';

export default class AccountListItem extends LightningElement {
   @api index;
   @api item;

   get isMod5() {
     return this.index % 5 == 0;
   }
}

Option 2 You use a wired function instead of a wired property. And then do some fun stuff with a changed data model. Although this is hacky.

import {LightningElement, wire, track} from 'lwc';
import getAccounts from '@salesforce/apex/TestController.getAccounts';

export default class AccountList extends LightningElement {

    @wire(getAccounts)
    wiredAccounts({ error, data}) {
       if (error) {
         // do error handling, please
       } else if (data) {
         this.accounts = data.map((acct, index) => {
               return {
                  acct: acct,
                  isMod5: index % 5 == 0 ? true : false
               }
           })
       }
    }