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 $234.99   9.611%   $257.5749   $257.57   $257.55   $257.60  
  Purchase #2 $217.42   10.791%   $240.8818   $240.88   $240.90   $240.90  
  Purchase #3 $238.44   6.353%   $253.5881   $253.59   $253.60   $253.60  
  Purchase #4 $48.79   8.493%   $52.9337   $52.93   $52.95   $52.90  
  Purchase #5 $200.53   5.727%   $212.0144   $212.01   $212.00   $212.00  
  Purchase #6 $154.88   13.767%   $176.2023   $176.20   $176.20   $176.20  
  Purchase #7 $138.46   9.786%   $152.0097   $152.01   $152.00   $152.00  
  7 purchase totals = $1,233.51       $1,345.2049   $1,345.19   $1,345.20   $1,345.20