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 $63.71   3.740%   $66.0928   $66.09   $66.10   $66.10  
  Purchase #2 $17.65   10.453%   $19.495   $19.50   $19.50   $19.50  
  Purchase #3 $56.67   5.613%   $59.8509   $59.85   $59.85   $59.90  
  Purchase #4 $219.44   10.512%   $242.5075   $242.51   $242.50   $242.50  
  Purchase #5 $100.62   5.362%   $106.0152   $106.02   $106.00   $106.00  
  Purchase #6 $239.94   9.270%   $262.1824   $262.18   $262.20   $262.20  
  Purchase #7 $240.49   9.252%   $262.7401   $262.74   $262.75   $262.70  
  7 purchase totals = $938.52       $1,018.8839   $1,018.89   $1,018.90   $1,018.90