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 $28.26   6.791%   $30.1791   $30.18   $30.20   $30.20  
  Purchase #2 $21.31   12.864%   $24.0513   $24.05   $24.05   $24.10  
  Purchase #3 $228.39   11.221%   $254.0176   $254.02   $254.00   $254.00  
  Purchase #4 $138.47   12.634%   $155.9643   $155.96   $155.95   $156.00  
  Purchase #5 $29.44   6.591%   $31.3804   $31.38   $31.40   $31.40  
  Purchase #6 $210.73   3.526%   $218.1603   $218.16   $218.15   $218.20  
  Purchase #7 $182.98   12.793%   $206.3886   $206.39   $206.40   $206.40  
  7 purchase totals = $839.58       $920.1416   $920.14   $920.15   $920.30