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 $25.80   9.384%   $28.2211   $28.22   $28.20   $28.20  
  Purchase #2 $27.75   12.338%   $31.1738   $31.17   $31.15   $31.20  
  Purchase #3 $189.73   7.317%   $203.6125   $203.61   $203.60   $203.60  
  Purchase #4 $149.82   3.604%   $155.2195   $155.22   $155.20   $155.20  
  Purchase #5 $212.30   8.818%   $231.0206   $231.02   $231.00   $231.00  
  Purchase #6 $124.36   9.117%   $135.6979   $135.70   $135.70   $135.70  
  Purchase #7 $122.42   6.859%   $130.8168   $130.82   $130.80   $130.80  
  7 purchase totals = $852.18       $915.7622   $915.76   $915.65   $915.70