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 $14.75   11.336%   $16.4221   $16.42   $16.40   $16.40  
  Purchase #2 $149.64   6.557%   $159.4519   $159.45   $159.45   $159.50  
  Purchase #3 $155.91   8.446%   $169.0782   $169.08   $169.10   $169.10  
  Purchase #4 $32.72   4.400%   $34.1597   $34.16   $34.15   $34.20  
  Purchase #5 $125.89   13.850%   $143.3258   $143.33   $143.35   $143.30  
  Purchase #6 $171.37   10.279%   $188.9851   $188.99   $189.00   $189.00  
  Purchase #7 $99.28   10.241%   $109.4473   $109.45   $109.45   $109.40  
  7 purchase totals = $749.56       $820.8701   $820.88   $820.90   $820.90