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 $70.51   9.511%   $77.2162   $77.22   $77.20   $77.20  
  Purchase #2 $139.33   4.415%   $145.4814   $145.48   $145.50   $145.50  
  Purchase #3 $121.60   7.704%   $130.9681   $130.97   $130.95   $131.00  
  Purchase #4 $100.47   12.362%   $112.8901   $112.89   $112.90   $112.90  
  Purchase #5 $125.99   10.645%   $139.4016   $139.40   $139.40   $139.40  
  Purchase #6 $169.71   4.628%   $177.5642   $177.56   $177.55   $177.60  
  Purchase #7 $5.40   13.505%   $6.1293   $6.13   $6.15   $6.10  
  7 purchase totals = $733.01       $789.6509   $789.65   $789.65   $789.70