It depends. If you do smart training and improve over time towards your goal/peak race, then yes, it will be easy to stay under 5. If you have already been doing a lot of hard training for quite a while leading up to your sub-5 performance, then it is definitely possible to stay under 5 and quite likely, but it won't be easy.
Should you run the day before a meet? I would say yes. How long depends on how many miles you have been doing. If you have only bee running 20-30 miles a week, then I would only do 1.5-2 miles with a couple very short brisk pickups thrown in. Nothing too taxing. If you have been doing a lot more mileage than 20-30 then you can stand to go an extra mile or so.
All of that depends if the race is important or not. If it is a regular meet in the middle of season, I wouldn't be afraid to run 3 miles or more if it's easy for you.