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 $81.81   9.804%   $89.8307   $89.83   $89.85   $89.80  
  Purchase #2 $181.81   9.514%   $199.1074   $199.11   $199.10   $199.10  
  Purchase #3 $214.34   7.550%   $230.5227   $230.52   $230.50   $230.50  
  Purchase #4 $218.25   13.355%   $247.3973   $247.40   $247.40   $247.40  
  Purchase #5 $106.73   8.480%   $115.7807   $115.78   $115.80   $115.80  
  Purchase #6 $113.77   3.564%   $117.8248   $117.82   $117.80   $117.80  
  Purchase #7 $138.38   8.539%   $150.1963   $150.20   $150.20   $150.20  
  7 purchase totals = $1,055.09       $1,150.6599   $1,150.66   $1,150.65   $1,150.60