Quote from Ghost of Cutten:
I'll explain why it's mistaken to do this, regardless of trading style. First, mental calculation takes longer than just looking at the P&L column in your spreadsheet. Secondly, it's less accurate. Even if you are out by only 0.1% per day, that is an error range of 5% each month (60% each year!). Third, you need to check your statements daily in case of errors (or unauthorised account withdrawals, excess/mistaken broker fees etc) anyway. Fourth, if and when you put on more complex positions like options or spreads, they are much harder to calculate mentally. Finally, even if you can calculate your P&L to perfection, it is always superior to have a way of double-checking that. Systems with backups, double (or triple) checks and failsafes are always more robust than those without.
I agree with not staring at P&L during the day, if you are a longer-term investor/trader. But even for long-term holdings, you need to check the price once each day, as a signal. If one of your stocks is up or down a significant amount one day (e.g. 5%+), then there is probably some news (and at the very least, the price action has changed a lot), and you need to double check to see if it could have further price impact. What if your biggest holding just had its finance director or auditor resign? What if a stock you are monitoring just got a take-over bid? BP or Olympus holders who didn't watch the price of their stock each day certainly paid for that laziness and complacency.
Checking your statements daily is an essential task of any serious market participant. Not bothering to do it is like a racer not bothering to check his engine oil, a skydiver not checking the packing of his parachute. It cannot help in any way, and can only cost you in the long run. Sure, it's always tempting to be lazy, and I bet everyone has backslided sometimes on these kind of things, but anyone who is serious about their trading needs to make it a habit and stick to it.