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 $224.86   11.177%   $249.9926   $249.99   $250.00   $250.00  
  Purchase #2 $195.67   12.352%   $219.8392   $219.84   $219.85   $219.80  
  Purchase #3 $49.55   12.411%   $55.6997   $55.70   $55.70   $55.70  
  Purchase #4 $112.85   5.300%   $118.8311   $118.83   $118.85   $118.80  
  Purchase #5 $223.60   11.653%   $249.6561   $249.66   $249.65   $249.70  
  Purchase #6 $93.97   7.659%   $101.1672   $101.17   $101.15   $101.20  
  Purchase #7 $139.29   7.794%   $150.1463   $150.15   $150.15   $150.10  
  7 purchase totals = $1,039.79       $1,145.3322   $1,145.34   $1,145.35   $1,145.30