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 $162.74   12.601%   $183.2469   $183.25   $183.25   $183.20  
  Purchase #2 $195.82   10.217%   $215.8269   $215.83   $215.85   $215.80  
  Purchase #3 $142.80   11.494%   $159.2134   $159.21   $159.20   $159.20  
  Purchase #4 $236.33   9.596%   $259.0082   $259.01   $259.00   $259.00  
  Purchase #5 $59.25   11.647%   $66.1508   $66.15   $66.15   $66.20  
  Purchase #6 $179.49   3.129%   $185.1062   $185.11   $185.10   $185.10  
  Purchase #7 $243.70   7.234%   $261.3293   $261.33   $261.35   $261.30  
  7 purchase totals = $1,220.13       $1,329.8817   $1,329.89   $1,329.90   $1,329.80