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 $81.51   9.261%   $89.0586   $89.06   $89.05   $89.10  
  Purchase #2 $72.30   10.710%   $80.0433   $80.04   $80.05   $80.00  
  Purchase #3 $229.97   5.760%   $243.2163   $243.22   $243.20   $243.20  
  Purchase #4 $37.69   8.382%   $40.8492   $40.85   $40.85   $40.80  
  Purchase #5 $14.59   12.593%   $16.4273   $16.43   $16.45   $16.40  
  Purchase #6 $138.49   6.470%   $147.4503   $147.45   $147.45   $147.50  
  Purchase #7 $159.71   13.266%   $180.8971   $180.90   $180.90   $180.90  
  7 purchase totals = $734.26       $797.9421   $797.95   $797.95   $797.90