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 $234.79   12.761%   $264.7516   $264.75   $264.75   $264.80  
  Purchase #2 $48.66   11.658%   $54.3328   $54.33   $54.35   $54.30  
  Purchase #3 $175.83   13.310%   $199.233   $199.23   $199.25   $199.20  
  Purchase #4 $139.77   4.811%   $146.4943   $146.49   $146.50   $146.50  
  Purchase #5 $225.97   7.522%   $242.9675   $242.97   $242.95   $243.00  
  Purchase #6 $157.76   6.253%   $167.6247   $167.62   $167.60   $167.60  
  Purchase #7 $157.35   4.112%   $163.8202   $163.82   $163.80   $163.80  
  7 purchase totals = $1,140.13       $1,239.2241   $1,239.21   $1,239.20   $1,239.20