I've tried that, but it gets tough to maintain any sort of balance over time. In a Spanish campaign, the Venetians overran Milan almost immediately and started to whack the French, so I allied with France and bankrolled the HRE to counter them. Fifteen turns later the Venetians were getting swarmed and my erstwhile allies the French were far stronger and on my borders. Soon after the HRE suffered a few defeats at the hands of the French, and their few remaining generals were so spoiled and disloyal they got bribed away leaving a bunch of rebel factions.

Anyway, if you manage to keep the peace, let me know how you did it!