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 $231.48   11.848%   $258.9058   $258.91   $258.90   $258.90  
  Purchase #2 $39.86   7.741%   $42.9456   $42.95   $42.95   $42.90  
  Purchase #3 $115.72   8.794%   $125.8964   $125.90   $125.90   $125.90  
  Purchase #4 $199.87   6.102%   $212.0661   $212.07   $212.05   $212.10  
  Purchase #5 $147.96   3.413%   $153.0099   $153.01   $153.00   $153.00  
  Purchase #6 $165.79   6.110%   $175.9198   $175.92   $175.90   $175.90  
  Purchase #7 $58.59   12.474%   $65.8985   $65.90   $65.90   $65.90  
  7 purchase totals = $959.27       $1,034.6421   $1,034.66   $1,034.60   $1,034.60