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 $244.92   12.162%   $274.7072   $274.71   $274.70   $274.70  
  Purchase #2 $94.75   9.341%   $103.6006   $103.60   $103.60   $103.60  
  Purchase #3 $21.97   12.193%   $24.6488   $24.65   $24.65   $24.60  
  Purchase #4 $123.68   8.317%   $133.9665   $133.97   $133.95   $134.00  
  Purchase #5 $8.50   12.187%   $9.5359   $9.54   $9.55   $9.50  
  Purchase #6 $62.33   6.626%   $66.46   $66.46   $66.45   $66.50  
  Purchase #7 $202.41   10.177%   $223.0093   $223.01   $223.00   $223.00  
  7 purchase totals = $758.56       $835.9283   $835.94   $835.90   $835.90