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 $28.75   4.331%   $29.9952   $30.00   $30.00   $30.00  
  Purchase #2 $230.63   11.517%   $257.1917   $257.19   $257.20   $257.20  
  Purchase #3 $77.32   7.369%   $83.0177   $83.02   $83.00   $83.00  
  Purchase #4 $102.82   13.848%   $117.0585   $117.06   $117.05   $117.10  
  Purchase #5 $78.29   11.500%   $87.2934   $87.29   $87.30   $87.30  
  Purchase #6 $139.96   4.140%   $145.7543   $145.75   $145.75   $145.80  
  Purchase #7 $182.55   10.770%   $202.2106   $202.21   $202.20   $202.20  
  7 purchase totals = $840.32       $922.5214   $922.52   $922.50   $922.60