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 $79.52   11.402%   $88.5869   $88.59   $88.60   $88.60  
  Purchase #2 $67.51   3.423%   $69.8209   $69.82   $69.80   $69.80  
  Purchase #3 $153.99   7.323%   $165.2667   $165.27   $165.25   $165.30  
  Purchase #4 $27.91   5.105%   $29.3348   $29.33   $29.35   $29.30  
  Purchase #5 $202.33   3.605%   $209.624   $209.62   $209.60   $209.60  
  Purchase #6 $168.40   4.113%   $175.3263   $175.33   $175.35   $175.30  
  Purchase #7 $47.65   6.224%   $50.6157   $50.62   $50.60   $50.60  
  7 purchase totals = $747.31       $788.5753   $788.58   $788.55   $788.50