Hi,
There is no magic formula here. However, your total Big Bank holdings should be considered in part of your broader portfolio diversification. It is my opinion that Canada's Big Five can be comfortably bought at any time. Each have their history of outperformance and each has spent time picking up the rear.
When I am due to top up my Big Bank holdings, I never just top up the ones I own - I always look to see which looks the most attractive at the time I am ready to buy. This involves taking a look at valuation, income and expected growth rates. I then simply buy what I deem to be the best bank at that point in time.
There is nothing wrong with splitting up and buying all five. At that point however, you must also take into account trading fees. One $5,000 purchase is only one transaction, whereas you would have to pay trading fees on 5 separate purchases if you were to split the $5k into $1K each. Buying an ETF is also an option to spread your investment across all of the Big Five. Once again however, you must take MER fees into account and you likely will not benefit from as high a yield, or be the full beneficiary of each dividend raise.
Things to consider.
Thanks
Mat