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 $244.33   9.166%   $266.7253   $266.73   $266.75   $266.70  
  Purchase #2 $180.97   4.281%   $188.7173   $188.72   $188.70   $188.70  
  Purchase #3 $144.32   10.861%   $159.9946   $159.99   $160.00   $160.00  
  Purchase #4 $34.52   4.251%   $35.9874   $35.99   $36.00   $36.00  
  Purchase #5 $94.83   12.495%   $106.679   $106.68   $106.70   $106.70  
  Purchase #6 $53.54   13.862%   $60.9617   $60.96   $60.95   $61.00  
  Purchase #7 $170.76   6.175%   $181.3044   $181.30   $181.30   $181.30  
  7 purchase totals = $923.27       $1,000.3697   $1,000.37   $1,000.40   $1,000.40