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 $129.58   6.553%   $138.0714   $138.07   $138.05   $138.10  
  Purchase #2 $178.29   10.704%   $197.3742   $197.37   $197.35   $197.40  
  Purchase #3 $8.55   10.566%   $9.4534   $9.45   $9.45   $9.50  
  Purchase #4 $216.76   6.566%   $230.9925   $230.99   $231.00   $231.00  
  Purchase #5 $124.37   5.756%   $131.5287   $131.53   $131.55   $131.50  
  Purchase #6 $90.78   12.354%   $101.995   $102.00   $102.00   $102.00  
  Purchase #7 $54.75   5.857%   $57.9567   $57.96   $57.95   $58.00  
  7 purchase totals = $803.08       $867.3719   $867.37   $867.35   $867.50