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 $123.39   13.513%   $140.0637   $140.06   $140.05   $140.10  
  Purchase #2 $17.27   8.869%   $18.8017   $18.80   $18.80   $18.80  
  Purchase #3 $26.32   12.627%   $29.6434   $29.64   $29.65   $29.60  
  Purchase #4 $10.37   8.799%   $11.2825   $11.28   $11.30   $11.30  
  Purchase #5 $136.31   8.817%   $148.3285   $148.33   $148.35   $148.30  
  Purchase #6 $58.85   3.431%   $60.8691   $60.87   $60.85   $60.90  
  Purchase #7 $212.57   8.732%   $231.1316   $231.13   $231.15   $231.10  
  7 purchase totals = $585.08       $640.1205   $640.11   $640.15   $640.10