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 $154.46   6.841%   $165.0266   $165.03   $165.05   $165.00  
  Purchase #2 $165.65   10.348%   $182.7915   $182.79   $182.80   $182.80  
  Purchase #3 $98.66   12.363%   $110.8573   $110.86   $110.85   $110.90  
  Purchase #4 $220.62   5.697%   $233.1887   $233.19   $233.20   $233.20  
  Purchase #5 $21.43   13.609%   $24.3464   $24.35   $24.35   $24.30  
  Purchase #6 $7.51   5.553%   $7.927   $7.93   $7.95   $7.90  
  Purchase #7 $35.48   10.187%   $39.0943   $39.09   $39.10   $39.10  
  7 purchase totals = $703.81       $763.2318   $763.24   $763.30   $763.20