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 $59.86   12.179%   $67.1503   $67.15   $67.15   $67.20  
  Purchase #2 $130.72   11.555%   $145.8247   $145.82   $145.80   $145.80  
  Purchase #3 $168.88   4.723%   $176.8562   $176.86   $176.85   $176.90  
  Purchase #4 $37.45   3.585%   $38.7926   $38.79   $38.80   $38.80  
  Purchase #5 $25.33   5.226%   $26.6537   $26.65   $26.65   $26.70  
  Purchase #6 $248.80   8.836%   $270.784   $270.78   $270.80   $270.80  
  Purchase #7 $142.53   7.495%   $153.2126   $153.21   $153.20   $153.20  
  7 purchase totals = $813.57       $879.2741   $879.26   $879.25   $879.40