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 $63.54   4.566%   $66.4412   $66.44   $66.45   $66.40  
  Purchase #2 $106.29   11.245%   $118.2423   $118.24   $118.25   $118.20  
  Purchase #3 $230.80   5.163%   $242.7162   $242.72   $242.70   $242.70  
  Purchase #4 $198.66   12.738%   $223.9653   $223.97   $223.95   $224.00  
  Purchase #5 $118.48   11.445%   $132.04   $132.04   $132.05   $132.00  
  Purchase #6 $35.90   9.236%   $39.2157   $39.22   $39.20   $39.20  
  Purchase #7 $241.91   6.517%   $257.6753   $257.68   $257.70   $257.70  
  7 purchase totals = $995.58       $1,080.2960   $1,080.31   $1,080.30   $1,080.20