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 $227.92   11.579%   $254.3109   $254.31   $254.30   $254.30  
  Purchase #2 $91.38   13.708%   $103.9064   $103.91   $103.90   $103.90  
  Purchase #3 $54.37   7.285%   $58.3309   $58.33   $58.35   $58.30  
  Purchase #4 $156.81   5.601%   $165.5929   $165.59   $165.60   $165.60  
  Purchase #5 $16.74   7.346%   $17.9697   $17.97   $17.95   $18.00  
  Purchase #6 $133.40   4.362%   $139.2189   $139.22   $139.20   $139.20  
  Purchase #7 $92.28   6.601%   $98.3714   $98.37   $98.35   $98.40  
  7 purchase totals = $772.90       $837.7011   $837.70   $837.65   $837.70