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 $245.34   12.453%   $275.8922   $275.89   $275.90   $275.90  
  Purchase #2 $116.97   6.735%   $124.8479   $124.85   $124.85   $124.80  
  Purchase #3 $44.83   11.573%   $50.0182   $50.02   $50.00   $50.00  
  Purchase #4 $32.59   9.791%   $35.7809   $35.78   $35.80   $35.80  
  Purchase #5 $100.35   7.470%   $107.8461   $107.85   $107.85   $107.80  
  Purchase #6 $22.35   12.777%   $25.2057   $25.21   $25.20   $25.20  
  Purchase #7 $238.59   5.507%   $251.7292   $251.73   $251.75   $251.70  
  7 purchase totals = $801.02       $871.3202   $871.33   $871.35   $871.20