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 $99.75   3.380%   $103.1216   $103.12   $103.10   $103.10  
  Purchase #2 $131.90   12.588%   $148.5036   $148.50   $148.50   $148.50  
  Purchase #3 $155.76   8.626%   $169.1959   $169.20   $169.20   $169.20  
  Purchase #4 $13.69   9.867%   $15.0408   $15.04   $15.05   $15.00  
  Purchase #5 $27.79   12.680%   $31.3138   $31.31   $31.30   $31.30  
  Purchase #6 $155.88   4.743%   $163.2734   $163.27   $163.25   $163.30  
  Purchase #7 $37.64   13.742%   $42.8125   $42.81   $42.80   $42.80  
  7 purchase totals = $622.41       $673.2616   $673.25   $673.20   $673.20