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 $39.90   11.234%   $44.3824   $44.38   $44.40   $44.40  
  Purchase #2 $138.98   8.583%   $150.9087   $150.91   $150.90   $150.90  
  Purchase #3 $115.36   12.196%   $129.4293   $129.43   $129.45   $129.40  
  Purchase #4 $233.94   12.868%   $264.0434   $264.04   $264.05   $264.00  
  Purchase #5 $165.40   8.607%   $179.636   $179.64   $179.65   $179.60  
  Purchase #6 $91.53   9.551%   $100.272   $100.27   $100.25   $100.30  
  Purchase #7 $180.68   3.285%   $186.6153   $186.62   $186.60   $186.60  
  7 purchase totals = $965.79       $1,055.2871   $1,055.29   $1,055.30   $1,055.20