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 $92.78   6.671%   $98.9694   $98.97   $98.95   $99.00  
  Purchase #2 $183.41   12.263%   $205.9016   $205.90   $205.90   $205.90  
  Purchase #3 $119.28   13.659%   $135.5725   $135.57   $135.55   $135.60  
  Purchase #4 $206.68   13.319%   $234.2077   $234.21   $234.20   $234.20  
  Purchase #5 $144.92   4.143%   $150.924   $150.92   $150.90   $150.90  
  Purchase #6 $65.52   11.642%   $73.1478   $73.15   $73.15   $73.10  
  Purchase #7 $99.35   5.640%   $104.9533   $104.95   $104.95   $105.00  
  7 purchase totals = $911.94       $1,003.6763   $1,003.67   $1,003.60   $1,003.70