Most of the times, Sellers would hesitate to put the house up for sale before summer; the reason being their perception of summer listings fetching a higher price for their home. This cannot be totally true.
Yes, the house prices are fractionally more in summer but not always so, and also it is due to a number of factors. For example, summer pricing is usually based on price precedent having been set through the previous winter and spring market sales. So a higher price is a natural progression of the the housing market.
So instead of dwelling on the hope of getting a higher price, don't delay your plans. And keep in mind that if you are going to get a higher price, you'll most likely be paying a higher price too for your next house.
Always talk to your Realtor for advice and information.