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 $9.87   4.798%   $10.3436   $10.34   $10.35   $10.30  
  Purchase #2 $91.48   9.864%   $100.5036   $100.50   $100.50   $100.50  
  Purchase #3 $129.89   5.696%   $137.2885   $137.29   $137.30   $137.30  
  Purchase #4 $212.50   3.703%   $220.3689   $220.37   $220.35   $220.40  
  Purchase #5 $36.68   11.249%   $40.8061   $40.81   $40.80   $40.80  
  Purchase #6 $100.42   8.501%   $108.9567   $108.96   $108.95   $109.00  
  Purchase #7 $53.61   3.787%   $55.6402   $55.64   $55.65   $55.60  
  7 purchase totals = $634.45       $673.9076   $673.91   $673.90   $673.90