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 $109.65   13.281%   $124.2126   $124.21   $124.20   $124.20  
  Purchase #2 $82.37   13.696%   $93.6514   $93.65   $93.65   $93.70  
  Purchase #3 $92.88   8.329%   $100.616   $100.62   $100.60   $100.60  
  Purchase #4 $65.54   13.740%   $74.5452   $74.55   $74.55   $74.50  
  Purchase #5 $235.42   6.787%   $251.398   $251.40   $251.40   $251.40  
  Purchase #6 $229.61   12.445%   $258.185   $258.19   $258.20   $258.20  
  Purchase #7 $174.82   6.848%   $186.7917   $186.79   $186.80   $186.80  
  7 purchase totals = $990.29       $1,089.3999   $1,089.41   $1,089.40   $1,089.40