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 $29.97   4.144%   $31.212   $31.21   $31.20   $31.20  
  Purchase #2 $83.83   7.831%   $90.3947   $90.39   $90.40   $90.40  
  Purchase #3 $204.63   13.102%   $231.4406   $231.44   $231.45   $231.40  
  Purchase #4 $204.62   11.547%   $228.2475   $228.25   $228.25   $228.20  
  Purchase #5 $26.64   13.558%   $30.2519   $30.25   $30.25   $30.30  
  Purchase #6 $120.66   13.539%   $136.9962   $137.00   $137.00   $137.00  
  Purchase #7 $102.33   7.374%   $109.8758   $109.88   $109.90   $109.90  
  7 purchase totals = $772.68       $858.4187   $858.42   $858.45   $858.40