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 $238.73   12.805%   $269.2994   $269.30   $269.30   $269.30  
  Purchase #2 $60.84   6.612%   $64.8627   $64.86   $64.85   $64.90  
  Purchase #3 $134.68   13.489%   $152.847   $152.85   $152.85   $152.80  
  Purchase #4 $152.29   5.600%   $160.8182   $160.82   $160.80   $160.80  
  Purchase #5 $149.49   10.425%   $165.0743   $165.07   $165.05   $165.10  
  Purchase #6 $203.40   9.750%   $223.2315   $223.23   $223.25   $223.20  
  Purchase #7 $56.80   11.421%   $63.2871   $63.29   $63.30   $63.30  
  7 purchase totals = $996.23       $1,099.4202   $1,099.42   $1,099.40   $1,099.40