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 $230.99   8.238%   $250.019   $250.02   $250.00   $250.00  
  Purchase #2 $74.54   4.471%   $77.8727   $77.87   $77.85   $77.90  
  Purchase #3 $152.56   6.809%   $162.9478   $162.95   $162.95   $162.90  
  Purchase #4 $118.29   5.870%   $125.2336   $125.23   $125.25   $125.20  
  Purchase #5 $10.49   13.196%   $11.8743   $11.87   $11.85   $11.90  
  Purchase #6 $242.65   6.291%   $257.9151   $257.92   $257.90   $257.90  
  Purchase #7 $145.67   4.679%   $152.4859   $152.49   $152.50   $152.50  
  7 purchase totals = $975.19       $1,038.3484   $1,038.35   $1,038.30   $1,038.30