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 $58.33   13.150%   $66.0004   $66.00   $66.00   $66.00  
  Purchase #2 $86.59   3.553%   $89.6665   $89.67   $89.65   $89.70  
  Purchase #3 $233.35   5.502%   $246.1889   $246.19   $246.20   $246.20  
  Purchase #4 $202.63   5.295%   $213.3593   $213.36   $213.35   $213.40  
  Purchase #5 $240.66   4.501%   $251.4921   $251.49   $251.50   $251.50  
  Purchase #6 $155.29   5.591%   $163.9723   $163.97   $163.95   $164.00  
  Purchase #7 $9.53   12.499%   $10.7212   $10.72   $10.70   $10.70  
  7 purchase totals = $986.38       $1,041.4007   $1,041.40   $1,041.35   $1,041.50