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 $242.35   3.573%   $251.0092   $251.01   $251.00   $251.00  
  Purchase #2 $50.54   12.301%   $56.7569   $56.76   $56.75   $56.80  
  Purchase #3 $23.44   7.463%   $25.1893   $25.19   $25.20   $25.20  
  Purchase #4 $62.36   9.221%   $68.1102   $68.11   $68.10   $68.10  
  Purchase #5 $29.55   7.760%   $31.8431   $31.84   $31.85   $31.80  
  Purchase #6 $210.25   11.522%   $234.475   $234.48   $234.50   $234.50  
  Purchase #7 $238.87   8.373%   $258.8706   $258.87   $258.85   $258.90  
  7 purchase totals = $857.36       $926.2543   $926.26   $926.25   $926.30