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 $224.26   11.279%   $249.5543   $249.55   $249.55   $249.60  
  Purchase #2 $55.66   13.659%   $63.2626   $63.26   $63.25   $63.30  
  Purchase #3 $39.90   6.773%   $42.6024   $42.60   $42.60   $42.60  
  Purchase #4 $102.41   13.224%   $115.9527   $115.95   $115.95   $116.00  
  Purchase #5 $218.38   4.105%   $227.3445   $227.34   $227.35   $227.30  
  Purchase #6 $150.27   13.408%   $170.4182   $170.42   $170.40   $170.40  
  Purchase #7 $150.53   12.469%   $169.2996   $169.30   $169.30   $169.30  
  7 purchase totals = $941.41       $1,038.4343   $1,038.42   $1,038.40   $1,038.50