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 $170.98   8.378%   $185.3047   $185.30   $185.30   $185.30  
  Purchase #2 $187.46   13.645%   $213.0389   $213.04   $213.05   $213.00  
  Purchase #3 $83.77   8.820%   $91.1585   $91.16   $91.15   $91.20  
  Purchase #4 $74.70   11.359%   $83.1852   $83.19   $83.20   $83.20  
  Purchase #5 $40.84   7.842%   $44.0427   $44.04   $44.05   $44.00  
  Purchase #6 $14.63   6.381%   $15.5635   $15.56   $15.55   $15.60  
  Purchase #7 $6.95   6.616%   $7.4098   $7.41   $7.40   $7.40  
  7 purchase totals = $579.33       $639.7033   $639.70   $639.70   $639.70