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 $55.63   6.493%   $59.2421   $59.24   $59.25   $59.20  
  Purchase #2 $227.56   8.778%   $247.5352   $247.54   $247.55   $247.50  
  Purchase #3 $174.31   12.375%   $195.8809   $195.88   $195.90   $195.90  
  Purchase #4 $144.93   13.186%   $164.0405   $164.04   $164.05   $164.00  
  Purchase #5 $52.46   11.145%   $58.3067   $58.31   $58.30   $58.30  
  Purchase #6 $232.63   5.608%   $245.6759   $245.68   $245.70   $245.70  
  Purchase #7 $65.48   5.344%   $68.9793   $68.98   $69.00   $69.00  
  7 purchase totals = $953.00       $1,039.6606   $1,039.67   $1,039.75   $1,039.60