Based on advice I heard from others - no. Sales are sales to you, so why limit someone's performance? Just be smart and don't structure commission based on leads (people will learn to game it) and only on actual sales, or at least limit leads-based compensation. Also, consider cancellations/churn as well to encourage healthy sales, perhaps pay bonuses for % of customers who stay on for X months or similar.
You shouldn't cap commissions if you're a startup! That's the key incentive for sales people for joining startups. Startups are risky by nature. The main appeal for sales people to join startups is the allure of generating a ton in sales commissions based on their ability to sell.
That said, most mid to large sized businesses do have caps. But that's something to figure out when your business reaches that level.