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 $19.77   7.409%   $21.2348   $21.23   $21.25   $21.20  
  Purchase #2 $73.30   8.523%   $79.5474   $79.55   $79.55   $79.50  
  Purchase #3 $210.30   10.837%   $233.0902   $233.09   $233.10   $233.10  
  Purchase #4 $117.30   3.211%   $121.0665   $121.07   $121.05   $121.10  
  Purchase #5 $228.33   4.338%   $238.235   $238.24   $238.25   $238.20  
  Purchase #6 $101.99   8.630%   $110.7917   $110.79   $110.80   $110.80  
  Purchase #7 $94.57   10.150%   $104.1689   $104.17   $104.15   $104.20  
  7 purchase totals = $845.56       $908.1345   $908.14   $908.15   $908.10