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 $17.86   5.488%   $18.8402   $18.84   $18.85   $18.80  
  Purchase #2 $47.48   9.852%   $52.1577   $52.16   $52.15   $52.20  
  Purchase #3 $21.66   12.520%   $24.3718   $24.37   $24.35   $24.40  
  Purchase #4 $38.51   12.311%   $43.251   $43.25   $43.25   $43.30  
  Purchase #5 $221.39   7.496%   $237.9854   $237.99   $238.00   $238.00  
  Purchase #6 $113.96   10.266%   $125.6591   $125.66   $125.65   $125.70  
  Purchase #7 $28.70   12.112%   $32.1761   $32.18   $32.20   $32.20  
  7 purchase totals = $489.56       $534.4413   $534.45   $534.45   $534.60