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 $238.95   10.653%   $264.4053   $264.41   $264.40   $264.40  
  Purchase #2 $117.79   10.673%   $130.3617   $130.36   $130.35   $130.40  
  Purchase #3 $54.81   6.636%   $58.4472   $58.45   $58.45   $58.40  
  Purchase #4 $149.52   8.613%   $162.3982   $162.40   $162.40   $162.40  
  Purchase #5 $98.26   7.567%   $105.6953   $105.70   $105.70   $105.70  
  Purchase #6 $140.30   6.301%   $149.1403   $149.14   $149.15   $149.10  
  Purchase #7 $123.99   5.236%   $130.4821   $130.48   $130.50   $130.50  
  7 purchase totals = $923.62       $1,000.9301   $1,000.94   $1,000.95   $1,000.90