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 $20.45   3.707%   $21.2081   $21.21   $21.20   $21.20  
  Purchase #2 $36.41   8.650%   $39.5595   $39.56   $39.55   $39.60  
  Purchase #3 $11.84   4.737%   $12.4009   $12.40   $12.40   $12.40  
  Purchase #4 $171.37   8.752%   $186.3683   $186.37   $186.35   $186.40  
  Purchase #5 $157.55   5.335%   $165.9553   $165.96   $165.95   $166.00  
  Purchase #6 $156.73   12.473%   $176.2789   $176.28   $176.30   $176.30  
  Purchase #7 $78.79   13.335%   $89.2966   $89.30   $89.30   $89.30  
  7 purchase totals = $633.14       $691.0676   $691.08   $691.05   $691.20