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 $121.56   6.502%   $129.4638   $129.46   $129.45   $129.50  
  Purchase #2 $90.77   4.554%   $94.9037   $94.90   $94.90   $94.90  
  Purchase #3 $190.97   4.747%   $200.0353   $200.04   $200.05   $200.00  
  Purchase #4 $9.47   5.310%   $9.9729   $9.97   $9.95   $10.00  
  Purchase #5 $57.52   6.182%   $61.0759   $61.08   $61.10   $61.10  
  Purchase #6 $227.42   10.117%   $250.4281   $250.43   $250.45   $250.40  
  Purchase #7 $157.93   7.506%   $169.7842   $169.78   $169.80   $169.80  
  7 purchase totals = $855.64       $915.6639   $915.66   $915.70   $915.70