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 $17.33   3.806%   $17.9896   $17.99   $18.00   $18.00  
  Purchase #2 $96.38   10.364%   $106.3688   $106.37   $106.35   $106.40  
  Purchase #3 $202.78   12.160%   $227.438   $227.44   $227.45   $227.40  
  Purchase #4 $12.72   11.569%   $14.1916   $14.19   $14.20   $14.20  
  Purchase #5 $89.96   4.817%   $94.2934   $94.29   $94.30   $94.30  
  Purchase #6 $188.49   7.132%   $201.9331   $201.93   $201.95   $201.90  
  Purchase #7 $20.65   9.692%   $22.6514   $22.65   $22.65   $22.70  
  7 purchase totals = $628.31       $684.8659   $684.86   $684.90   $684.90