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 $81.47   10.451%   $89.9844   $89.98   $90.00   $90.00  
  Purchase #2 $77.58   8.520%   $84.1898   $84.19   $84.20   $84.20  
  Purchase #3 $33.41   9.213%   $36.4881   $36.49   $36.50   $36.50  
  Purchase #4 $123.98   11.675%   $138.4547   $138.45   $138.45   $138.50  
  Purchase #5 $53.81   12.332%   $60.4458   $60.45   $60.45   $60.40  
  Purchase #6 $120.35   6.724%   $128.4423   $128.44   $128.45   $128.40  
  Purchase #7 $157.68   5.684%   $166.6425   $166.64   $166.65   $166.60  
  7 purchase totals = $648.28       $704.6476   $704.64   $704.70   $704.60