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 $27.43   9.323%   $29.9873   $29.99   $30.00   $30.00  
  Purchase #2 $87.33   6.641%   $93.1296   $93.13   $93.15   $93.10  
  Purchase #3 $77.91   8.747%   $84.7248   $84.72   $84.70   $84.70  
  Purchase #4 $214.50   8.867%   $233.5197   $233.52   $233.50   $233.50  
  Purchase #5 $223.55   10.724%   $247.5235   $247.52   $247.50   $247.50  
  Purchase #6 $80.57   9.833%   $88.4924   $88.49   $88.50   $88.50  
  Purchase #7 $67.70   8.330%   $73.3394   $73.34   $73.35   $73.30  
  7 purchase totals = $778.99       $850.7167   $850.71   $850.70   $850.60