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 $17.28   7.165%   $18.5181   $18.52   $18.50   $18.50  
  Purchase #2 $18.25   7.375%   $19.5959   $19.60   $19.60   $19.60  
  Purchase #3 $101.88   6.360%   $108.3596   $108.36   $108.35   $108.40  
  Purchase #4 $152.31   12.750%   $171.7295   $171.73   $171.75   $171.70  
  Purchase #5 $42.76   5.261%   $45.0096   $45.01   $45.00   $45.00  
  Purchase #6 $91.59   7.655%   $98.6012   $98.60   $98.60   $98.60  
  Purchase #7 $201.68   4.143%   $210.0356   $210.04   $210.05   $210.00  
  7 purchase totals = $625.75       $671.8495   $671.86   $671.85   $671.80