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 $6.75   10.626%   $7.4673   $7.47   $7.45   $7.50  
  Purchase #2 $160.25   12.400%   $180.121   $180.12   $180.10   $180.10  
  Purchase #3 $8.56   11.780%   $9.5684   $9.57   $9.55   $9.60  
  Purchase #4 $134.92   10.189%   $148.667   $148.67   $148.65   $148.70  
  Purchase #5 $115.85   3.754%   $120.199   $120.20   $120.20   $120.20  
  Purchase #6 $21.74   13.239%   $24.6182   $24.62   $24.60   $24.60  
  Purchase #7 $78.46   12.618%   $88.3601   $88.36   $88.35   $88.40  
  7 purchase totals = $526.53       $579.0010   $579.01   $578.90   $579.10