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 $204.87   11.116%   $227.6433   $227.64   $227.65   $227.60  
  Purchase #2 $197.68   13.245%   $223.8627   $223.86   $223.85   $223.90  
  Purchase #3 $12.48   4.319%   $13.019   $13.02   $13.00   $13.00  
  Purchase #4 $103.35   6.427%   $109.9923   $109.99   $110.00   $110.00  
  Purchase #5 $158.55   10.723%   $175.5513   $175.55   $175.55   $175.60  
  Purchase #6 $226.70   8.166%   $245.2123   $245.21   $245.20   $245.20  
  Purchase #7 $6.79   13.853%   $7.7306   $7.73   $7.75   $7.70  
  7 purchase totals = $910.42       $1,003.0115   $1,003.00   $1,003.00   $1,003.00