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 $6.45   4.229%   $6.7228   $6.72   $6.70   $6.70  
  Purchase #2 $207.40   10.331%   $228.8265   $228.83   $228.85   $228.80  
  Purchase #3 $102.75   12.603%   $115.6996   $115.70   $115.70   $115.70  
  Purchase #4 $15.72   10.620%   $17.3895   $17.39   $17.40   $17.40  
  Purchase #5 $150.93   11.700%   $168.5888   $168.59   $168.60   $168.60  
  Purchase #6 $130.77   5.669%   $138.1834   $138.18   $138.20   $138.20  
  Purchase #7 $127.99   10.855%   $141.8833   $141.88   $141.90   $141.90  
  7 purchase totals = $742.01       $817.2939   $817.29   $817.35   $817.30