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 $108.72   13.801%   $123.7244   $123.72   $123.70   $123.70  
  Purchase #2 $202.92   10.624%   $224.4782   $224.48   $224.50   $224.50  
  Purchase #3 $55.68   3.349%   $57.5447   $57.54   $57.55   $57.50  
  Purchase #4 $232.86   5.709%   $246.154   $246.15   $246.15   $246.20  
  Purchase #5 $237.69   4.222%   $247.7253   $247.73   $247.75   $247.70  
  Purchase #6 $87.61   9.605%   $96.0249   $96.02   $96.00   $96.00  
  Purchase #7 $32.43   4.361%   $33.8443   $33.84   $33.85   $33.80  
  7 purchase totals = $957.91       $1,029.4958   $1,029.48   $1,029.50   $1,029.40