Let's do some math ..  Rounding examples of 7 random hypothetical purchases:
  Case Tag  
Price  
Sales  
Tax %  
Raw Total  
Incl. Sales Tax  
Rounded  
to Penny  
Rounded  
to Nickel  
Rounded  
to Dime  
  Purchase #1 $45.25   13.574%   $51.3922   $51.39   $51.40   $51.40  
  Purchase #2 $91.86   6.699%   $98.0137   $98.01   $98.00   $98.00  
  Purchase #3 $217.88   13.819%   $247.9888   $247.99   $248.00   $248.00  
  Purchase #4 $173.25   8.162%   $187.3907   $187.39   $187.40   $187.40  
  Purchase #5 $241.89   4.617%   $253.0581   $253.06   $253.05   $253.10  
  Purchase #6 $240.45   8.257%   $260.304   $260.30   $260.30   $260.30  
  Purchase #7 $71.54   11.843%   $80.0125   $80.01   $80.00   $80.00  
  7 purchase totals = $1,082.12       $1,178.1600   $1,178.15   $1,178.15   $1,178.20