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 $73.54   9.478%   $80.5101   $80.51   $80.50   $80.50  
  Purchase #2 $87.96   6.689%   $93.8436   $93.84   $93.85   $93.80  
  Purchase #3 $218.83   6.162%   $232.3143   $232.31   $232.30   $232.30  
  Purchase #4 $149.77   11.568%   $167.0954   $167.10   $167.10   $167.10  
  Purchase #5 $116.55   3.690%   $120.8507   $120.85   $120.85   $120.90  
  Purchase #6 $19.71   12.494%   $22.1726   $22.17   $22.15   $22.20  
  Purchase #7 $187.57   10.214%   $206.7284   $206.73   $206.75   $206.70  
  7 purchase totals = $853.93       $923.5151   $923.51   $923.50   $923.50