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 $7.38   8.370%   $7.9977   $8.00   $8.00   $8.00  
  Purchase #2 $19.27   4.643%   $20.1647   $20.16   $20.15   $20.20  
  Purchase #3 $6.82   8.623%   $7.4081   $7.41   $7.40   $7.40  
  Purchase #4 $158.54   3.470%   $164.0413   $164.04   $164.05   $164.00  
  Purchase #5 $68.90   7.256%   $73.8994   $73.90   $73.90   $73.90  
  Purchase #6 $111.66   12.645%   $125.7794   $125.78   $125.80   $125.80  
  Purchase #7 $154.71   11.364%   $172.2912   $172.29   $172.30   $172.30  
  7 purchase totals = $527.28       $571.5818   $571.58   $571.60   $571.60