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 $81.93   9.843%   $89.9944   $89.99   $90.00   $90.00  
  Purchase #2 $102.57   3.736%   $106.402   $106.40   $106.40   $106.40  
  Purchase #3 $135.67   11.214%   $150.884   $150.88   $150.90   $150.90  
  Purchase #4 $230.66   12.705%   $259.9654   $259.97   $259.95   $260.00  
  Purchase #5 $174.55   10.646%   $193.1326   $193.13   $193.15   $193.10  
  Purchase #6 $159.54   10.215%   $175.837   $175.84   $175.85   $175.80  
  Purchase #7 $192.65   11.541%   $214.8837   $214.88   $214.90   $214.90  
  7 purchase totals = $1,077.57       $1,191.0991   $1,191.09   $1,191.15   $1,191.10