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 $201.72   12.578%   $227.0923   $227.09   $227.10   $227.10  
  Purchase #2 $19.43   12.617%   $21.8815   $21.88   $21.90   $21.90  
  Purchase #3 $84.58   4.649%   $88.5121   $88.51   $88.50   $88.50  
  Purchase #4 $145.69   5.222%   $153.2979   $153.30   $153.30   $153.30  
  Purchase #5 $95.75   11.757%   $107.0073   $107.01   $107.00   $107.00  
  Purchase #6 $137.48   6.773%   $146.7915   $146.79   $146.80   $146.80  
  Purchase #7 $215.37   7.253%   $230.9908   $230.99   $231.00   $231.00  
  7 purchase totals = $900.02       $975.5734   $975.57   $975.60   $975.60