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 $18.94   10.768%   $20.9795   $20.98   $21.00   $21.00  
  Purchase #2 $63.39   5.682%   $66.9918   $66.99   $67.00   $67.00  
  Purchase #3 $47.87   10.787%   $53.0337   $53.03   $53.05   $53.00  
  Purchase #4 $142.88   8.331%   $154.7833   $154.78   $154.80   $154.80  
  Purchase #5 $189.50   11.774%   $211.8117   $211.81   $211.80   $211.80  
  Purchase #6 $225.68   8.810%   $245.5624   $245.56   $245.55   $245.60  
  Purchase #7 $132.67   10.576%   $146.7012   $146.70   $146.70   $146.70  
  7 purchase totals = $820.93       $899.8636   $899.85   $899.90   $899.90