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 $248.50   12.543%   $279.6694   $279.67   $279.65   $279.70  
  Purchase #2 $196.61   4.117%   $204.7044   $204.70   $204.70   $204.70  
  Purchase #3 $150.53   13.725%   $171.1902   $171.19   $171.20   $171.20  
  Purchase #4 $84.71   12.335%   $95.159   $95.16   $95.15   $95.20  
  Purchase #5 $141.54   11.710%   $158.1143   $158.11   $158.10   $158.10  
  Purchase #6 $123.52   3.254%   $127.5393   $127.54   $127.55   $127.50  
  Purchase #7 $206.84   4.380%   $215.8996   $215.90   $215.90   $215.90  
  7 purchase totals = $1,152.25       $1,252.2762   $1,252.27   $1,252.25   $1,252.30