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 $21.40   4.265%   $22.3127   $22.31   $22.30   $22.30  
  Purchase #2 $187.87   5.395%   $198.0056   $198.01   $198.00   $198.00  
  Purchase #3 $48.61   5.245%   $51.1596   $51.16   $51.15   $51.20  
  Purchase #4 $106.93   12.260%   $120.0396   $120.04   $120.05   $120.00  
  Purchase #5 $32.48   5.610%   $34.3021   $34.30   $34.30   $34.30  
  Purchase #6 $8.25   12.727%   $9.3   $9.30   $9.30   $9.30  
  Purchase #7 $96.55   10.509%   $106.6964   $106.70   $106.70   $106.70  
  7 purchase totals = $502.09       $541.8160   $541.82   $541.80   $541.80