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 $132.83   4.366%   $138.6294   $138.63   $138.65   $138.60  
  Purchase #2 $9.49   5.196%   $9.9831   $9.98   $10.00   $10.00  
  Purchase #3 $37.54   6.264%   $39.8915   $39.89   $39.90   $39.90  
  Purchase #4 $80.98   12.434%   $91.0491   $91.05   $91.05   $91.00  
  Purchase #5 $22.42   9.228%   $24.4889   $24.49   $24.50   $24.50  
  Purchase #6 $110.39   4.843%   $115.7362   $115.74   $115.75   $115.70  
  Purchase #7 $86.25   10.365%   $95.1898   $95.19   $95.20   $95.20  
  7 purchase totals = $479.90       $514.9680   $514.97   $515.05   $514.90