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 $244.39   12.304%   $274.4597   $274.46   $274.45   $274.50  
  Purchase #2 $166.30   9.354%   $181.8557   $181.86   $181.85   $181.90  
  Purchase #3 $161.49   6.190%   $171.4862   $171.49   $171.50   $171.50  
  Purchase #4 $198.64   8.735%   $215.9912   $215.99   $216.00   $216.00  
  Purchase #5 $161.42   12.693%   $181.909   $181.91   $181.90   $181.90  
  Purchase #6 $233.91   10.643%   $258.805   $258.81   $258.80   $258.80  
  Purchase #7 $206.87   5.643%   $218.5437   $218.54   $218.55   $218.50  
  7 purchase totals = $1,373.02       $1,503.0505   $1,503.06   $1,503.05   $1,503.10