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 $100.59   8.365%   $109.0044   $109.00   $109.00   $109.00  
  Purchase #2 $9.30   7.438%   $9.9917   $9.99   $10.00   $10.00  
  Purchase #3 $35.41   3.303%   $36.5796   $36.58   $36.60   $36.60  
  Purchase #4 $70.35   10.211%   $77.5334   $77.53   $77.55   $77.50  
  Purchase #5 $182.38   8.198%   $197.3315   $197.33   $197.35   $197.30  
  Purchase #6 $86.72   9.249%   $94.7407   $94.74   $94.75   $94.70  
  Purchase #7 $73.43   6.171%   $77.9614   $77.96   $77.95   $78.00  
  7 purchase totals = $558.18       $603.1427   $603.13   $603.20   $603.10