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 $180.43   3.840%   $187.3585   $187.36   $187.35   $187.40  
  Purchase #2 $75.88   6.744%   $80.9973   $81.00   $81.00   $81.00  
  Purchase #3 $71.93   8.852%   $78.2972   $78.30   $78.30   $78.30  
  Purchase #4 $31.79   11.577%   $35.4703   $35.47   $35.45   $35.50  
  Purchase #5 $152.63   5.553%   $161.1055   $161.11   $161.10   $161.10  
  Purchase #6 $135.96   6.528%   $144.8355   $144.84   $144.85   $144.80  
  Purchase #7 $241.63   3.113%   $249.1519   $249.15   $249.15   $249.20  
  7 purchase totals = $890.25       $937.2162   $937.23   $937.20   $937.30