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 $18.79   11.657%   $20.9804   $20.98   $21.00   $21.00  
  Purchase #2 $21.82   8.729%   $23.7247   $23.72   $23.70   $23.70  
  Purchase #3 $42.51   9.663%   $46.6177   $46.62   $46.60   $46.60  
  Purchase #4 $243.62   13.415%   $276.3016   $276.30   $276.30   $276.30  
  Purchase #5 $64.40   3.560%   $66.6926   $66.69   $66.70   $66.70  
  Purchase #6 $44.36   3.278%   $45.8141   $45.81   $45.80   $45.80  
  Purchase #7 $38.69   11.160%   $43.0078   $43.01   $43.00   $43.00  
  7 purchase totals = $474.19       $523.1389   $523.13   $523.10   $523.10