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 $185.75   12.146%   $208.3112   $208.31   $208.30   $208.30  
  Purchase #2 $145.97   4.702%   $152.8335   $152.83   $152.85   $152.80  
  Purchase #3 $235.60   6.224%   $250.2637   $250.26   $250.25   $250.30  
  Purchase #4 $203.68   9.693%   $223.4227   $223.42   $223.40   $223.40  
  Purchase #5 $241.54   4.219%   $251.7306   $251.73   $251.75   $251.70  
  Purchase #6 $67.29   11.706%   $75.167   $75.17   $75.15   $75.20  
  Purchase #7 $223.37   13.787%   $254.166   $254.17   $254.15   $254.20  
  7 purchase totals = $1,303.20       $1,415.8947   $1,415.89   $1,415.85   $1,415.90