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 $136.79   12.162%   $153.4264   $153.43   $153.45   $153.40  
  Purchase #2 $36.43   8.871%   $39.6617   $39.66   $39.65   $39.70  
  Purchase #3 $222.60   13.869%   $253.4724   $253.47   $253.45   $253.50  
  Purchase #4 $96.48   5.166%   $101.4642   $101.46   $101.45   $101.50  
  Purchase #5 $157.45   6.159%   $167.1473   $167.15   $167.15   $167.10  
  Purchase #6 $6.38   7.320%   $6.847   $6.85   $6.85   $6.80  
  Purchase #7 $102.55   7.204%   $109.9377   $109.94   $109.95   $109.90  
  7 purchase totals = $758.68       $831.9567   $831.96   $831.95   $831.90