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 $27.90   7.805%   $30.0776   $30.08   $30.10   $30.10  
  Purchase #2 $224.60   5.286%   $236.4724   $236.47   $236.45   $236.50  
  Purchase #3 $138.38   8.518%   $150.1672   $150.17   $150.15   $150.20  
  Purchase #4 $77.38   4.798%   $81.0927   $81.09   $81.10   $81.10  
  Purchase #5 $227.95   5.421%   $240.3072   $240.31   $240.30   $240.30  
  Purchase #6 $79.84   8.112%   $86.3166   $86.32   $86.30   $86.30  
  Purchase #7 $16.67   13.307%   $18.8883   $18.89   $18.90   $18.90  
  7 purchase totals = $792.72       $843.3220   $843.33   $843.30   $843.40