HI Olivier,
This answer will probably vary depending on who you speak to. Some like to keep large cash balances, others aim to keep a certain portion of their portfolio in cash (say 5%) and then there are those that keep no cash. I am in the latter category.
I keep very little cash in my accounts and typically deploy it whenever I have it and my cash balances usually end up being below 1%. I just believe that time in the market is better than time out, and that you can find something to buy regardless of market conditions. It's not for everyone, folks like to keep cash to take advantage of dips, or to as a 'defensive' mechanism.
I don't sell my stocks to raise cash. When I buy I stock, I usually buy with the intention to hold for the long-term. That means at least 3-5 year periods - so selling those positions to buy others - I don't do that often. Only in rare cases where my investment thesis changes, or perhaps I am looking to book gains according to a pre-determined plan.
Not sure if this helps, but that is how i managed cash.
Mat