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 $240.76   6.246%   $255.7979   $255.80   $255.80   $255.80  
  Purchase #2 $51.72   9.562%   $56.6655   $56.67   $56.65   $56.70  
  Purchase #3 $227.78   5.721%   $240.8113   $240.81   $240.80   $240.80  
  Purchase #4 $94.33   12.198%   $105.8364   $105.84   $105.85   $105.80  
  Purchase #5 $116.73   11.212%   $129.8178   $129.82   $129.80   $129.80  
  Purchase #6 $203.82   10.276%   $224.7645   $224.76   $224.75   $224.80  
  Purchase #7 $213.98   5.449%   $225.6398   $225.64   $225.65   $225.60  
  7 purchase totals = $1,149.12       $1,239.3332   $1,239.34   $1,239.30   $1,239.30