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 $17.38   8.677%   $18.8881   $18.89   $18.90   $18.90  
  Purchase #2 $143.52   12.483%   $161.4356   $161.44   $161.45   $161.40  
  Purchase #3 $137.76   11.249%   $153.2566   $153.26   $153.25   $153.30  
  Purchase #4 $134.75   10.667%   $149.1238   $149.12   $149.10   $149.10  
  Purchase #5 $7.59   7.497%   $8.159   $8.16   $8.15   $8.20  
  Purchase #6 $170.82   13.212%   $193.3887   $193.39   $193.40   $193.40  
  Purchase #7 $177.60   9.481%   $194.4383   $194.44   $194.45   $194.40  
  7 purchase totals = $789.42       $878.6901   $878.70   $878.70   $878.70