I ALWAYS wait to look at my cards until the action is on me, ring games, tourneys, etc;
People give off so many tells when they peak ahead of time:
Leaning forward and looking anxious (great hand)
Look at their chips (some do it so quickly it is easy to miss if you don't watch them which can happen if you are sneaking a peak at your cards) which indicates they want to raise
Grabbing a card cover and placing it over their cards (indicates they are going to be in the hand)
Grabbing chips in advance and holding them in their closed hand (have to watch carefully for how much they grab - are they limping or raising?)
Positioning their cards out away from them (indicates a fold)
Positioning their cards close to them (indicates they are staying in)
Lean back in their chair after looking (indicates a fold)
Look up at the tv or somewhere else (non interest, folding)
Look closely at other players (they are staying unless they are attempting to read others)
Tight squeeze of their cards (good hand)
Loosely drop their cards back down (bad hand)
Stop chewing their gum briefly (great hand)
Keep chewing their gum (okay to bad hand)
Shake their leg (very good hand)
Hands shake slightly for a moment (monster hand)
Start sweating (monster hand)
Wipe their hands on their pants (monster hand or difficult decision)
Let out a noticeable breath of air (monster hand or difficult decision hand)
Body tenses (great hand)
This list isn't complete and these aren't all 100% but once you have sized up the caliber of your opps then for the bad players or inexperienced ones it's pretty darn close.
I pick up on so many tells while playing, many of them happen so fast you really have to be watching the person. If they are wearing glasses it is harder as those who do a quick glance down towards their chips often don't move their head, just their eyes.
Watching others really helps me to make my decisions when I look at my cards. I can almost "insta-decide" what I am going to do as soon as I see my cards because I already have reads on some (sometimes many) of my opps. I might throw away my QJ su in the co because I know the nit in the sb is going to raise or reraise pf. I might throw out a healthy size raise in the co with my 24su because I know the blinds are going to fold if I do because of the way they looked at their cards and other tells they gave off.
For those that "sneak a peak" live - you have to play the way that is most comfortable for you. Just consider the above and stop and think if you do any of these things when you look before your turn. Players like you help me to make decisions when I look at my cards. Of course there are some who may give off fake tells, but not very many.