How would I round all the values in an excel sheet to two decimal places?
mrmcg,
=TEXT(A1,"0.00") is my preferred method here.
In hopes to be helpful it is a good idea to understand what you need to change to get the correct result. When you change the formatting (i.e. number, decimal, general) you are only changing the way the data appears, not the actual data.
You will need to use a formula like round, floor, text to change the actual data.
good luck and I hope that this is helpful!
The following will radically change your data so use it with caution; possibly even on a backup copy until you are sure this is what you want.
If you have all of the numbers formatted the way you want (e.g. 0.00) then go into Excel Options (Alt+F,T). Choose Advanced from the list down the left and scroll the right pane down ¾ of the way to the When calculating this workbook section.
Click Set precision as displayed. Acknowledge the warning.

Remember to also click OK in the lower right to accept the option change and close Excel Options. All of the numeric values within your workbook will be rounded off to their displayed value of their number format. Note that numbers set to a General should not be affected.
Use at your own risk. I'm not saying this is a bad thing; just that you should be completely aware of the consequences before you commit large amounts of data to this option. See Change formula recalculation, iteration, or precision for more information.