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 $74.59   6.615%   $79.5241   $79.52   $79.50   $79.50  
  Purchase #2 $180.93   13.206%   $204.8236   $204.82   $204.80   $204.80  
  Purchase #3 $5.61   4.759%   $5.877   $5.88   $5.90   $5.90  
  Purchase #4 $215.99   3.652%   $223.878   $223.88   $223.90   $223.90  
  Purchase #5 $208.86   12.813%   $235.6212   $235.62   $235.60   $235.60  
  Purchase #6 $125.54   13.343%   $142.2908   $142.29   $142.30   $142.30  
  Purchase #7 $87.49   4.871%   $91.7516   $91.75   $91.75   $91.80  
  7 purchase totals = $899.01       $983.7663   $983.76   $983.75   $983.80