First of all not ALL Clematis are hardy, there are quite a few which will only grow in a Conservatory or against a South facing wall in favoured areas. However C. macropetala is as tough as old boot leather. The only reason I can see for not planting out at present is if it has been kept in a warm environment to bring it into early growth for selling. In which case,yes wait until the soil has warmed up a little and frost is less likely.