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 $113.91   3.769%   $118.2033   $118.20   $118.20   $118.20  
  Purchase #2 $139.80   6.661%   $149.1121   $149.11   $149.10   $149.10  
  Purchase #3 $36.92   7.566%   $39.7134   $39.71   $39.70   $39.70  
  Purchase #4 $179.35   4.414%   $187.2665   $187.27   $187.25   $187.30  
  Purchase #5 $230.41   12.663%   $259.5868   $259.59   $259.60   $259.60  
  Purchase #6 $42.56   4.253%   $44.3701   $44.37   $44.35   $44.40  
  Purchase #7 $248.99   8.432%   $269.9848   $269.98   $270.00   $270.00  
  7 purchase totals = $991.94       $1,068.2370   $1,068.23   $1,068.20   $1,068.30