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 $234.66   6.773%   $250.5535   $250.55   $250.55   $250.60  
  Purchase #2 $58.82   5.147%   $61.8475   $61.85   $61.85   $61.80  
  Purchase #3 $13.34   12.160%   $14.9621   $14.96   $14.95   $15.00  
  Purchase #4 $59.57   5.590%   $62.9   $62.90   $62.90   $62.90  
  Purchase #5 $163.54   5.861%   $173.1251   $173.13   $173.15   $173.10  
  Purchase #6 $95.98   8.331%   $103.9761   $103.98   $104.00   $104.00  
  Purchase #7 $105.27   11.552%   $117.4308   $117.43   $117.45   $117.40  
  7 purchase totals = $731.18       $784.7951   $784.80   $784.85   $784.80