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 $181.45   12.340%   $203.8409   $203.84   $203.85   $203.80  
  Purchase #2 $244.72   12.405%   $275.0775   $275.08   $275.10   $275.10  
  Purchase #3 $28.28   11.867%   $31.636   $31.64   $31.65   $31.60  
  Purchase #4 $105.94   4.577%   $110.7889   $110.79   $110.80   $110.80  
  Purchase #5 $37.26   6.270%   $39.5962   $39.60   $39.60   $39.60  
  Purchase #6 $145.56   12.462%   $163.6997   $163.70   $163.70   $163.70  
  Purchase #7 $229.51   10.710%   $254.0905   $254.09   $254.10   $254.10  
  7 purchase totals = $972.72       $1,078.7297   $1,078.74   $1,078.80   $1,078.70