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 $242.51   13.527%   $275.3143   $275.31   $275.30   $275.30  
  Purchase #2 $171.33   9.106%   $186.9313   $186.93   $186.95   $186.90  
  Purchase #3 $35.44   12.632%   $39.9168   $39.92   $39.90   $39.90  
  Purchase #4 $113.58   5.821%   $120.1915   $120.19   $120.20   $120.20  
  Purchase #5 $118.45   13.255%   $134.1505   $134.15   $134.15   $134.20  
  Purchase #6 $55.73   12.355%   $62.6154   $62.62   $62.60   $62.60  
  Purchase #7 $58.86   5.651%   $62.1862   $62.19   $62.20   $62.20  
  7 purchase totals = $795.90       $881.3060   $881.31   $881.30   $881.30