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 $66.43   13.279%   $75.2512   $75.25   $75.25   $75.30  
  Purchase #2 $223.71   10.653%   $247.5418   $247.54   $247.55   $247.50  
  Purchase #3 $167.39   5.427%   $176.4743   $176.47   $176.45   $176.50  
  Purchase #4 $97.44   13.344%   $110.4424   $110.44   $110.45   $110.40  
  Purchase #5 $93.42   7.505%   $100.4312   $100.43   $100.45   $100.40  
  Purchase #6 $146.49   6.507%   $156.0221   $156.02   $156.00   $156.00  
  Purchase #7 $69.37   11.429%   $77.2983   $77.30   $77.30   $77.30  
  7 purchase totals = $864.25       $943.4613   $943.45   $943.45   $943.40