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 $243.91   6.318%   $259.3202   $259.32   $259.30   $259.30  
  Purchase #2 $194.96   6.121%   $206.8935   $206.89   $206.90   $206.90  
  Purchase #3 $222.55   8.439%   $241.331   $241.33   $241.35   $241.30  
  Purchase #4 $52.96   13.214%   $59.9581   $59.96   $59.95   $60.00  
  Purchase #5 $108.81   9.424%   $119.0643   $119.06   $119.05   $119.10  
  Purchase #6 $158.68   9.589%   $173.8958   $173.90   $173.90   $173.90  
  Purchase #7 $244.86   12.623%   $275.7687   $275.77   $275.75   $275.80  
  7 purchase totals = $1,226.73       $1,336.2316   $1,336.23   $1,336.20   $1,336.30