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 $243.93   13.408%   $276.6361   $276.64   $276.65   $276.60  
  Purchase #2 $158.98   10.802%   $176.153   $176.15   $176.15   $176.20  
  Purchase #3 $17.74   12.418%   $19.943   $19.94   $19.95   $19.90  
  Purchase #4 $237.26   9.268%   $259.2493   $259.25   $259.25   $259.20  
  Purchase #5 $147.97   8.218%   $160.1302   $160.13   $160.15   $160.10  
  Purchase #6 $107.35   5.774%   $113.5484   $113.55   $113.55   $113.50  
  Purchase #7 $6.97   7.830%   $7.5158   $7.52   $7.50   $7.50  
  7 purchase totals = $920.20       $1,013.1758   $1,013.18   $1,013.20   $1,013.00