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 $239.59   10.376%   $264.4499   $264.45   $264.45   $264.40  
  Purchase #2 $232.39   5.359%   $244.8438   $244.84   $244.85   $244.80  
  Purchase #3 $122.53   6.760%   $130.813   $130.81   $130.80   $130.80  
  Purchase #4 $63.28   5.632%   $66.8439   $66.84   $66.85   $66.80  
  Purchase #5 $127.40   5.191%   $134.0133   $134.01   $134.00   $134.00  
  Purchase #6 $41.61   4.135%   $43.3306   $43.33   $43.35   $43.30  
  Purchase #7 $48.37   4.439%   $50.5171   $50.52   $50.50   $50.50  
  7 purchase totals = $875.17       $934.8116   $934.80   $934.80   $934.60