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 $245.42   12.437%   $275.9429   $275.94   $275.95   $275.90  
  Purchase #2 $198.36   3.315%   $204.9356   $204.94   $204.95   $204.90  
  Purchase #3 $164.30   5.659%   $173.5977   $173.60   $173.60   $173.60  
  Purchase #4 $127.37   9.109%   $138.9721   $138.97   $138.95   $139.00  
  Purchase #5 $198.82   11.679%   $222.0402   $222.04   $222.05   $222.00  
  Purchase #6 $111.36   11.411%   $124.0673   $124.07   $124.05   $124.10  
  Purchase #7 $138.65   6.148%   $147.1742   $147.17   $147.15   $147.20  
  7 purchase totals = $1,184.28       $1,286.7300   $1,286.73   $1,286.70   $1,286.70